[llvm] [MLGO] Add tests for scripts (PR #78878)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

This does a couple additional dependencies, but they're relatively small, will already be present when running `check-all`, and actually fully exercises the common path of `extract_ir.py` which we weren't able to do previously in `ml-compiler-opt`.

https://github.com/llvm/llvm-project/pull/78878