tests: replace dead shim tripwires with a live no-coverage check

[cargo-affected-bot]

Problem

Five assertions across the two basename-collision scenarios grep stderr for strings the code no longer emits, so they are permanently true:

$ rg 'failed to resolve binary_id|basename fallback ambiguous' src/
$   # no matches

Both strings belonged to the pre-NEXTEST_BINARY_ID shim, which resolved binaries by path and probed byte markers to disambiguate colliding basenames. That resolver is gone — the shim now reads NEXTEST_BINARY_ID straight from the env (src/shim.rs#L115-L120) — but the assertions that pinned its behavior stayed behind. tests/CLAUDE.md calls silent under-selection "the one failure mode this tool cannot detect downstream"; these two scenarios are supposed to be part of that guard and currently aren't.

The gap is not theoretical, because a failure to resolve a binary_id is a soft failure. That test lands as TestOutcome::Skipped, and as long as some other binary produced rows, collect still exits 0 (src/collect.rs#L376-L379). So the surviving collect.status.success() assertion doesn't cover it either: a green collect whose database is missing one of the two colliding binaries is exactly the shape both scenarios exist to catch.

Fix

Point each assertion at "produced no coverage" — the line collect prints whenever any test is skipped — and drop the now-redundant second basename fallback ambiguous check. The module docs' used to bail with … history is untouched; those describe the old shim in the past tense and are accurate.

Verification

Per tests/CLAUDE.md ("a test written for a bug must be seen failing against the pre-fix behavior"), each new assertion was run against an injected break — a temporary branch in shim.rs returning Skipped for one of the two colliding binaries, simulating a resolution failure on exactly one target. Both scenarios failed on the new assertion; neither would have failed on the old one.

Injected-break output

Break applied in src/shim.rs (not committed):

let outcome = if binary_id.contains("wt_perf") {
    TestOutcome::Skipped { reason: "TEMP BREAK for tripwire verification".into() }
} else {
    extract(&dir, Path::new(binary), &env)
};

duplicate_basename_with_stripped_debuginfo_resolves_correctly — nextest reports both tests passing, collect exits 0, the database gets one test's rows instead of two, and the old string appears nowhere in stderr:

    Starting 2 tests across 4 binaries
        PASS [   0.007s] (1/2) dup_strip_wt_perf::builds builds
        PASS [   0.012s] (2/2) dup_strip_mock_stub::builds builds
     Summary [   0.012s] 2 tests run: 2 passed, 0 skipped
  skipped dup_strip_wt_perf::builds::builds: TEMP BREAK for tripwire verification
1 test produced no coverage
storing coverage for 1 tests (3 ranges)...
done. 1 tests, 3 ranges stored in target/affected/coverage.db (0.6s total)

panicked at tests/functional/duplicate_target_names.rs:87:5:
duplicate-basename binaries must not cost a test its coverage

lib_bin_same_basename_resolves_via_nextest_binary_id with the break narrowed to bin/wt-perf (both of its targets share the wt_perf_collide package, so the broader predicate tripped the hard mappings.is_empty() bail instead):

    Starting 2 tests across 2 binaries
        PASS [   0.007s] (1/2) wt_perf_collide::bin/wt-perf tests::bin_test_invokes_lib
        PASS [   0.012s] (2/2) wt_perf_collide tests::lib_test_double
     Summary [   0.012s] 2 tests run: 2 passed, 0 skipped
  skipped wt_perf_collide::bin/wt-perf::tests::bin_test_invokes_lib: TEMP BREAK …
1 test produced no coverage
storing coverage for 1 tests (3 ranges)...

With the break reverted, cargo test is green: 41 passed, 0 failed. cargo fmt --check and cargo clippy --all-targets both clean.

Overlap with open PRs

#31 and #63 both retouch the message on !stderr.contains("basename fallback ambiguous") in lib_bin_collision.rs — the assertion this PR deletes. Their other hunks (module docs, inline comments) are independent and unaffected. Whichever lands second will want a trivial conflict resolution on that hunk; if this one lands first, those hunks simply become moot.

[cargo-affected-bot]

Self-review. CI is green on all three platforms, and the core claim checks out: rg finds neither failed to resolve binary_id nor basename fallback ambiguous anywhere in src/, and Skipped is only ever constructed from real extraction failures in shim.rs — never for a test that merely executes nothing — so the new assertions can't false-positive on the empty builds() test.

One observation beyond the inline nit. The replacements are still string-greps against a collect message, the same shape as the two they retire: a future reword of eprintln!("{skipped} test{s} produced no coverage") in collect.rs puts all four assertions back in the permanently-true state this PR exists to fix, silently. On the first collect in both scenarios that's cushioned — the SELECT DISTINCT binary_id assertions below would fail independently if a target were dropped. The uncushioned spot is the second collect in each scenario (recollect): neither re-queries test_regions afterwards, so the stderr grep is the sole guard there and also the only genuinely new coverage this PR adds. Re-running the same binary_id query after the recollect would pin the property against message edits as well as against a dropped target.