[GitHub] Add basic CI for libclang Python binding unit tests (#76784)

This is important to aid development of Python type annotations in the
libclang binding.
See https://github.com/llvm/llvm-project/issues/76664 for more details.

* Run on all pull requests and direct pushes.
* This makes use of the existing llvm-project-tests.yml recipe, which
will preload ccache from previous runs.
* Building libclang currently takes about 9mins when ccache is warm and
about an 1hr 20mins if it is cold using the standard GitHub ubuntu
runner.
* In the future, this could be broken into the following discrete steps
for clarity:
   1. Build libclang dependency.
       ninja -C build libclang
   2. Run Python unit tests.
       ninja -C build check-clang-python
* Followup changes will bring testing on older python versions and
static type checking.

Issue https://github.com/llvm/llvm-project/issues/76601.