[Clang] [Python] Fix tests when default config file contains -include

In Gentoo, we make use of Clang's recently-enhanced config file support
and add a default include to `clang` invocations using '-include ...'.

This breaks clang-python tests like so:
```
======================================================================
ERROR: test_includes (tests.cindex.test_translation_unit.TestTranslationUnit)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/var/tmp/portage/dev-python/clang-python-15.0.6/work/clang/bindings/python/tests/cindex/test_translation_unit.py", line 145, in test_includes
    eq(i[0], i[1])
  File "/var/tmp/portage/dev-python/clang-python-15.0.6/work/clang/bindings/python/tests/cindex/test_translation_unit.py", line 132, in eq
    self.assert_normpaths_equal(expected[0], actual.source.name)
AttributeError: 'NoneType' object has no attribute 'name'

======================================================================
FAIL...