Add option to instrument which functions are used by which test

[bjorn3]

See https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Fwg-rls-2.2E0/topic/Live.20unit.20testing/near/217883005

With lazy compilation instrumenting would be trivial. It wouldn't even be necessary to add instrumentation calls in the jitted code. Just record the function call when you lazily compile a function.
You could reset the GOT used for swapping all calls from the lazy compilation stub to the jitted code back to the lazy compilation stubs after every test and then as optimization keep a pointer to the jitted code in a side-table.