[llvm] [CI] Refactor generate_test_report script (PR #133196)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> (and btw, don't feel the need to keep any of this code if it would be easier to junk it and use something custom or a built-in GitHub feature if you find one)

For now this seems like the path of least resistance. We could do things a bit nicer in Github, but there are tradeoffs, mainly with parallelism if we want to split the job into multiple steps that each invoke their own test targets.

https://github.com/llvm/llvm-project/pull/133196