[clang][python][test] Move python binding tests to lit framework (#145855)

As discussed in PR #142353, the current testsuite of the `clang` Python
bindings has several issues:

- If `libclang.so` cannot be loaded into `python` to run the testsuite,
the whole `ninja check-all` aborts.
- The result of running the testsuite isn't report like the `lit`-based
tests, rendering them almost invisible.
- The testsuite is disabled in a non-obvious way (`RUN_PYTHON_TESTS`) in
`tests/CMakeLists.txt`, which again doesn't show up in the test results.

All these issues can be avoided by integrating the Python bindings tests
with `lit`, which is what this patch does:

- The actual test lives in `clang/test/bindings/python/bindings.sh` and
is run by `lit`.
- The current `clang/bindings/python/tests` directory (minus the
now-superfluous `CMakeLists.txt`) is moved into the same directory.
- The check if `libclang` is loadable (originally from PR #142353) is
now handled via a new `lit` feature, `libclang-loadable`.
- The various ways to disable the tests have been turned into `XFAIL`s
as appropriate.
- AArch64 doesn't `FAIL` any longer, so no `XFAIL` is necessary.
- It keeps the `check-clang-python` target for use by the Clang Python
CI.

Tested on `sparc-sun-solaris2.11`, `sparcv9-sun-solaris2.11`,
`i386-pc-solaris2.11`, `amd64-pc-solaris2.11`, `i686-pc-linux-gnu`, and
`x86_64-pc-linux-gnu`.

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