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

[Rainer Orth via llvm-commits]

rorth wrote:

> > am at a bit of a loss how to handle build_target now [...] Don't know if/how I can test this myself.
> 
> All the `check-clang-python` did is depend on the `libclang` target and then call unittest, so I think it should be enough to change the `build_target` to `libclang`, and then add another step to the workflow that calls `lit` as you described. Feel free to abuse the CI to test this until it works.

I've made some adjustments.  The weird thing is that `build_target` isn't even documented in [Workflow syntax for GitHub Actions](https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions).  I've added an `llvm-lit` invocation that should run the test, but don't know anything about the layout of the build tree.  Attempts to test this may have to wait until I return from vacation in a week and a half.

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