[clang] [libclang/python] Improve test coverage (PR #109846)

[Jannick Kremer via cfe-commits]

https://github.com/DeinAlptraum commented:

@Endilll this adds the tests in preparation as discussed for #105490. While I was at it, I also added full test coverage for all the affected classes.

https://github.com/llvm/llvm-project/pull/109846