src/test/run-make*/coverage* tests should be reimplemented as a custom compiletest

[richkadel]

The coverage tests (for -Z instrument-coverage) are implemented using Makefiles and embedded shell scripts, which is fragile.

Some cross-platform issues are unavoidable, but there have been some cross-platform issues, due to differences in the shell scripts and external commands (like sed).

Also, there is a known bug in some versions of make on MacOS that corrupts shell scripts during script interpretation.

Some of these issues could be avoided in the future if the test logic was rewritten/ported to Rust as a compiletest.

[richkadel]

@tmandry

[richkadel]

@rustbot +A-code-coverage

[Zalathar]

I was bitten pretty badly by cross-platform differences in #110942/#111179, so I'm investigating what it would take to integrate the coverage tests into compiletest as a new test category.

[Zalathar]

There is still one test left in tests/run-make/coverage-llvmir, but that's much less complex than the coverage-reports tests that were migrated.

It mostly delegates to $(LLVM_FILECHECK) anyway, so it seems fine as-is for now.