sagemathgh-40558: Add long time marker to several slow tests
    
This gets rid of about half of the warnings, until someone figure out
whether they're intended to be slow, or how they can be sped up.

Reference: sagemath#39569,
sagemath#39746

### 📝 Checklist

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- [ ] The title is concise and informative.
- [ ] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

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<!-- - sagemath#12345: short description why this is a dependency -->
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URL: sagemath#40558
Reported by: user202729
Reviewer(s): Michael Orlitzky, user202729

Author: Release Manager

src/sage/algebras/lie_conformal_algebras/n2_lie_conformal_algebra.py

@@ -80,7 +80,7 @@ def __init__(self, R):
         TESTS::
 
             sage: V = lie_conformal_algebras.N2(QQ)
-            sage: TestSuite(V).run()
+            sage: TestSuite(V).run()  # long time (:issue:`39569`)
         """
         n2dict = {('L', 'L'): {0: {('L', 1): 1},
                                1: {('L', 0): 2},

src/sage/geometry/hyperplane_arrangement/ordered_arrangement.py

@@ -400,13 +400,13 @@ def projective_fundamental_group(self):
             (0, 0, 0, 0, 0)
             sage: A4.<t1, t2, t3, t4> = OrderedHyperplaneArrangements(QQ)
             sage: H = A4(hyperplane_arrangements.braid(4))
-            sage: G4 = H.projective_fundamental_group(); G4.sorted_presentation()
+            sage: G4 = H.projective_fundamental_group(); G4.sorted_presentation()  # long time (:issue:`39569`)
             Finitely presented group
             < x0, x1, x2, x3, x4 | x4^-1*x3^-1*x2^-1*x3*x4*x0*x2*x0^-1,
                                    x4^-1*x2^-1*x4*x2, x4^-1*x1^-1*x0^-1*x1*x4*x0,
                                    x4^-1*x1^-1*x0^-1*x4*x0*x1,
                                    x3^-1*x2^-1*x1^-1*x0^-1*x3*x0*x1*x2, x3^-1*x1^-1*x3*x1 >
-            sage: G4.abelian_invariants()
+            sage: G4.abelian_invariants()  # long time (:issue:`39569`)
             (0, 0, 0, 0, 0)
 
             sage: # needs sirocco

src/sage/graphs/graph_decompositions/modular_decomposition.pyx

@@ -84,7 +84,7 @@ def corneil_habib_paul_tedder_algorithm(G):
         ....:                                                         3, 4, 0.2)
         sage: recreate_decomposition(10, corneil_habib_paul_tedder_algorithm,
         ....:                                                         4, 5, 0.2)
-        sage: recreate_decomposition(3, corneil_habib_paul_tedder_algorithm,
+        sage: recreate_decomposition(3, corneil_habib_paul_tedder_algorithm,  # long time (:issue:`39569`)
         ....:                                                         6, 5, 0.2)
 
         sage: H = Graph('Hv|mmjz', format='graph6')

src/sage/graphs/independent_sets.pyx

@@ -161,7 +161,7 @@ cdef class IndependentSets:
             ....:         IS2.extend(map(Set, list(G.subgraph_search_iterator(Graph(n), induced=True, return_graphs=False))))
             ....:     if len(IS) != len(set(IS2)):
             ....:        raise ValueError("something goes wrong")
-            sage: for i in range(5):                                                    # needs sage.modules
+            sage: for i in range(5):                                                    # needs sage.modules, long time (:issue:`39569`)
             ....:     check_with_subgraph_search(graphs.RandomGNP(11, .3))
 
         Empty graph::

src/sage/interfaces/maxima_abstract.py

@@ -304,7 +304,7 @@ def _commands(self, verbose=True):
         EXAMPLES::
 
             # The output is kind of random
-            sage: sorted(maxima._commands(verbose=False))
+            sage: sorted(maxima._commands(verbose=False))  # long time (:issue:`39569`)
             [...
              'display',
              ...

src/sage/libs/singular/polynomial.pyx

@@ -270,6 +270,7 @@ cdef int singular_polynomial_cmp(poly *p, poly *q, ring *r) noexcept:
 
     ::
 
+        sage: # long time (:issue:`39569`)
         sage: R.<x,y> = Integers(10)[]
         sage: l = [i*x+j*y+k for i in range(10) for j in range(10) for k in range(10)]
         sage: l.sort()

src/sage/modular/modform/element.py

@@ -2760,6 +2760,7 @@ def _pow_int(self, n):
 
         Testing modular forms of nontrivial character::
 
+            sage: # long time (:issue:`39569`)
             sage: F = ModularForms(DirichletGroup(17).0^2, 2).2
             sage: F3 = F^3; F3
             q^3 + (-3*zeta8^2 + 6)*q^4 + (-12*zeta8^2 + 3*zeta8 + 18)*q^5 + O(q^6)

src/sage/rings/function_field/khuri_makdisi.pyx

@@ -820,6 +820,7 @@ cdef class KhuriMakdisi_small(KhuriMakdisi_base):
 
         Check that :issue:`40237` is fixed::
 
+            sage: # long time (:issue:`39569`)
             sage: K = GF(2)
             sage: F.<x> = FunctionField(K)
             sage: t = polygen(F)
@@ -873,6 +874,7 @@ cdef class KhuriMakdisi_small(KhuriMakdisi_base):
 
         Check that :issue:`39148` is fixed::
 
+            sage: # long time (:issue:`39569`)
             sage: k.<x> = FunctionField(GF(17)); t = polygen(k)
             sage: F.<y> = k.extension(t^4 + (14*x + 14)*t^3 + 9*t^2 + (10*x^2 + 15*x + 8)*t
             ....:  + 7*x^3 + 15*x^2 + 6*x + 16)

src/sage/rings/number_field/number_field_element.pyx

@@ -507,7 +507,7 @@ cdef class NumberFieldElement(NumberFieldElement_base):
 
         Check that :issue:`15276` is fixed::
 
-            sage: for n in range(2,20):                                                 # needs sage.libs.gap
+            sage: for n in range(2,20):                                                 # needs sage.libs.gap, long time (:issue:`39569`)
             ....:     K = CyclotomicField(n)
             ....:     assert K(gap(K.gen())) == K.gen(), "n = {}".format(n)
             ....:     assert K(gap(K.one())) == K.one(), "n = {}".format(n)

src/sage/rings/semirings/tropical_mpolynomial.py

@@ -543,6 +543,7 @@ def dual_subdivision(self):
 
         A subdivision with many faces, not all of which are triangles::
 
+            sage: # long time (:issue:`39569`)
             sage: T = TropicalSemiring(QQ)
             sage: R.<x,y> = PolynomialRing(T)
             sage: p3 = (R(8) + R(4)*x + R(2)*y + R(1)*x^2 + x*y + R(1)*y^2
@@ -578,6 +579,7 @@ def dual_subdivision(self):
 
         Dual subdivision of a tropical surface::
 
+            sage: # long time (:issue:`39569`)
             sage: T = TropicalSemiring(QQ)
             sage: R.<x,y,z> = PolynomialRing(T)
             sage: p1 = x + y + z + x^2 + R(1)