[clang/python] Add Python libclang unit tests to CI

[linux4life798]

It doesn't look like any GitHub workflows ever run the libclang Python binding unittests (target check-clang-python).

This should be added to the clang CI to promote ease of development.

[linux4life798]

A basic test to add this can be found in https://github.com/linux4life798/llvm-project/blob/add-python-workflow-checks/.github/workflows/clang-tests.yml#L39.

[linux4life798]

It looks like the check-all target, which is built/run from llvm-tests.yml, does include the check-clang-python target. I think we should still have this test explicitly run in the clang specific tests (clang-tests.yml) and also add the last few supported Python versions to the test matrix.

The following output is from https://github.com/llvm/llvm-project/actions/runs/6703627427/job/18214521420:

...
[4639/5636] cd /home/runner/work/llvm-project/llvm-project/clang/bindings/python && /usr/local/bin/cmake -E env CLANG_NO_DEFAULT_CONFIG=1 CLANG_LIBRARY_PATH=/home/runner/work/llvm-project/llvm-project/build/lib /opt/hostedtoolcache/Python/3.11.6/x64/bin/python3.11 -m unittest discover
.................................................................................................................................
----------------------------------------------------------------------
Ran 129 tests in 1.390s

OK
...

[linux4life798]

There is something wrong with the TranslationUnit unittest on Windows.
See the error in https://github.com/linux4life798/llvm-project/actions/runs/7372545686/job/20060970297.

FAIL: test_unsaved_files (tests.cindex.test_translation_unit.TestTranslationUnit)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\a\llvm-project\llvm-project\clang\bindings\python\tests\cindex\test_translation_unit.py", line 105, in test_unsaved_files

    self.assertEqual(spellings[-1], "y")

AssertionError: 'SOME_DEFINE' != 'y'

- SOME_DEFINE

+ y