bootstrap: Allow path-based skipping of the coverage test suite

[Zalathar]

• Alternative to bootstrap: honor suite skips for coverage modes #159079

---

When bootstrap runs a step by default, without explicit paths, it will normally act as though the user had explicitly requested all of the paths and aliases that the step registered through ShouldRun.

For the coverage test suite, that gives the wrong outcome. Running a command like ./x test --skip=tests or ./x test --skip=coverage should skip the coverage tests, but instead the coverage tests would run anyway, due to the coverage-map and coverage-run aliases being treated as implied command-line arguments.

This commit fixes that problem by adding a special flag to ShouldRun. When creating pathsets for a step that is being run by default, if the step has set the default_to_suites_only flag, all non-suite pathsets are discarded. That gives the desired behaviour for skipping coverage tests, without affecting other steps, since other steps don't set the flag.

The end result is that ./x test --skip=tests should now skip the coverage tests, as intended. This lets us remove some --skip arguments from CI scripts, which were only required by the previous incorrect behaviour.

---

The default_to_suites_only flag is a bit of a hack, but to me it seems like the cleanest way to resolve this problem without having to completely overhaul how CLI paths work, which is a much bigger task. And I think being able to remove the weird extra --skip arguments from CI scripts makes this a net positive.

r? jieyouxu

[Zalathar]

Running a few try jobs to double-check that coverage jobs are still skipped in the appropriate CI jobs:

@bors try jobs=x86_64-msvc-2,x86_64-gnu-llvm-22-1

[rust-bors]

☀️ Try build successful (CI)
Build commit: 6bcad16 (6bcad167e4ed6a807b18096e765a3d0ac0979ccd)
Base parent: 3b58636 (3b58636b30eb364ac72aeaf03d46347084ed87d1)