[clang] [llvm] [clang][python][test] Move python binding tests to lit framework (PR #142948)

[Rainer Orth via cfe-commits]

rorth wrote:

> I still don't entirely understand how lit works exactly, but it seems to me that completely removing the cmake target for this is not necessary, no? So at least this wouldn't require the CI changes on top of the other changes. Happy to hear your thoughts on this though @rorth

It occured to me that restoring a `clang-check-python` target might be the easiest solution:

- I'd originally removed it because the python tests would now be run by regular `ninja check-clang`, not knowing about the CI and its use there.
- Given how incredibly hard it seems to be to teach the CI to run just a single command (and me having no experience with GitHub Actions and seeing no need for them in the forseeable future), modifying the workflow doesn't seem doable to me.
- Besides, while I'm by no means a `cmake` expert, I find my way around (and can test things locally).

So I've reintroruced the target so the workflow should run unchanged, now only being `lit`-based.  I've exluded it from `ninja check-all` for fear of it running twice then, once from `check-clang` and then by itself.

I've retested this version on the same set of platforms as before (with the exception of Linux/sparc64 which is slow) and manually run `ninja check-clang-python`, all with the expected results.

Let me know what you think.

https://github.com/llvm/llvm-project/pull/142948