coverage: Register test::Coverage as the test suite for tests/coverage

This restores the ability to run a coverage test by specifying its path, e.g.
`./x.py test tests/coverage/if.rs`. This runs the test in both modes.

src/bootstrap/src/core/build_steps/test.rs

@@ -1423,7 +1423,7 @@ impl Step for Coverage {
     const ONLY_HOSTS: bool = false;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        run.alias(Self::SUITE)
+        run.suite_path(Self::PATH)
     }
 
     fn make_run(run: RunConfig<'_>) {
@@ -1438,6 +1438,7 @@ impl Step for Coverage {
     }
 }
 
+// Aliases for running the coverage tests in only one mode.
 coverage_test_alias!(CoverageMap {
     alias_and_mode: "coverage-map",
     default: true,

tests/coverage/README.md

@@ -1,13 +1,16 @@
-Many of these tests were copied from `tests/run-coverage` in order to
-capture the current behavior of the instrumentor on non-trivial programs.
-The actual mappings have not been closely inspected.
+The tests in this directory are shared by two different test modes, and can be
+run in multiple different ways:
+
+- `./x.py test coverage-map` (compiles to LLVM IR and checks coverage mappings)
+- `./x.py test run-coverage` (runs a test binary and checks its coverage report)
+- `./x.py test coverage` (runs both `coverage-map` and `run-coverage`)
 
 ## Maintenance note
 
 These tests can be sensitive to small changes in MIR spans or MIR control flow,
 especially in HIR-to-MIR lowering or MIR optimizations.
 
-If you haven't touched the coverage code directly, and the `run-coverage` test
-suite still works, then it should usually be OK to just `--bless` these
-coverage mapping tests as necessary, without worrying too much about the exact
-changes.
+If you haven't touched the coverage code directly, and the tests still pass in
+`run-coverage` mode, then it should usually be OK to just re-bless the mappings
+as necessary with `./x.py test coverage-map --bless`, without worrying too much
+about the exact changes.