Document how to check what tests each CI job run

[marcoieni]

CI jobs are defined in the jobs.yml file. We need to document how to find out which tests each job run.

E.g. people should be able to answer questions like:

I want to run tests/codegen on an i686 mingw try job, which CI job should I be using?".

Document this in https://rustc-dev-guide.rust-lang.org/tests/ci.html

For reference, the initial zulip request is here.

Related work

• Make it easier to run CI jobs locally rust#134898