[clang] [clang][python][test] Check if libclang.so is loadable (PR #142353)

[Rainer Orth via cfe-commits]

rorth wrote:

FWIW I now have a prototype of the python tests within the `lit` framework.  The basics (`PASS` if `python` and `libclang.so` match, `FAIL` if not) work, but there's more to be done:
- The test should be `UNSUPPORTED` when `libclang.so` cannot be loaded.
- The various ways `RUN_PYTHON_TESTS` can be set to `FALSE` need to be done with `XFAIL:` or `UNSUPPORTED:` in the new `lit` test.

https://github.com/llvm/llvm-project/pull/142353