sage.matrix.matrix_space: Rename a test_... function to _test_... (with deprecation)

[mkoeppe]

(split out from #33549)

Various Sage modules contain non-private helper functions that are only used in TESTS:: blocks of doctests of the same module.
Example:

• https://doc.sagemath.org/html/en/reference/matrices/sage/matrix/matrix_space.html#sage.matrix.matrix_space.test_trivial_matrices_inverse

They clutter the documentation and their module's namespace. We rename them (with deprecation) to _test_....
Thus they will no longer appear in the built documentation unless SAGE_DOC_UNDERSCORE is set.

CC: @tscrim @tobiasdiez

Component: documentation

Author: Tobias Diez, Matthias Koeppe

Branch/Commit: 5598464

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/33612

[mkoeppe]

Author: Tobias Diez, Matthias Koeppe

[mkoeppe]

Branch: u/mkoeppe/rename_some_test_____functions_to__test______with_deprecation_

[mkoeppe]

New commits:

4fa9546	src/sage/matrix/matrix_space.py: Make test method private
5598464	src/sage/matrix/matrix_space.py: Add deprecated_function_alias test_trivial_matrices_inverse

[mkoeppe]

Commit: 5598464

[tscrim]

comment:4

LGTM (patchbot failure seems unrelated).

[tscrim]

Reviewer: Travis Scrimshaw

[mkoeppe]

comment:5

Thanks!

[vbraun]

Changed branch from u/mkoeppe/rename_some_test_____functions_to__test______with_deprecation_ to 5598464