Run KNOWNBUG and THOROUGH regression tests in CI

Use the check-ubuntu-20_04-cmake-gcc job to run KNOWNBUG (any test
reported as failure will tell us that a bug has unexpectedly been fixed)
an THOROUGH (tests that are expected to pass, but take longer to do so)
tests.

Author: tautschnig

.github/workflows/pull-request-checks.yaml

@@ -162,7 +162,7 @@ jobs:
           ninja package
           ls *.deb
       - name: Run tests
-        run: cd build; ctest . -V -L CORE -j2
+        run: cd build; ctest . -V -L '(THOROUGH|KNOWNBUG)' -j2
 
   check-macos-10_15-make-clang:
     runs-on: macos-10.15