compiletest: add self-instrumentation for time taken between different steps

[jieyouxu]

E.g. for PRs like #136404, it would be very useful to have self-instrumentation for the time taken between steps (e.g. test discovery/collection, directive parsing, etc.). The compiler has self-instrumentation infra that we can steal adapt for our purposes.

Marked as E-hard because it can be annoying to implement this cleanly in the current code structure (there is no phase-split between discovery -> directive parsing -> test running).