[CI] Refactor generate_test_report script

[boomanaiden154]

This patch refactors the generate_test_report script, namely turning it
into a proper library, and pulling the script/unittests out into
separate files, as is standard with most python scripts. The main
purpose of this is to enable reusing the library for the new Github
premerge.

[DavidSpickett]

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)

[boomanaiden154]

(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.