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

[David Spickett via llvm-commits]

https://github.com/DavidSpickett approved this pull request.

LGTM.

I was thinking why don't you have the same entry point for GitHub and Buildkite as for now, the only difference would be that the environment variables wouldn't be set.

But that is not the only difference, in buildkite there is an agent to send the report to. There is not in Github.

So I like splitting this so you can have a GitHub entry point, as you do in the follow up PR.

(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)

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