[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: test_inclusion_directive (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 157, in test_inclusion_directive
    self.assert_normpaths_equal(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 126, in assert_normpaths_equal
    self.assertEqual(os.path.normpath(path1),
AssertionError: '/var/tmp/portage/dev-python/clang-python-1[58 chars]r1.h' != '/usr/include/gentoo/fortify.h'
- /var/tmp/portage/dev-python/clang-python-15.0.6/work/clang/bindings/python/tests/cindex/INPUTS/header1.h
+ /usr/include/gentoo/fortify.h
```

Disable using the default Clang configuration files on the system, like
we did for other tests.

Bug: https://bugs.gentoo.org/890204
Differential Revision: https://reviews.llvm.org/D141248