Four suites in this workspace were found hollow in one week, each hollow in a
different way, and every one of them had read as fine for months. The pattern
is not a bug in any of them — it is that a test suite reports on the code and
nothing reports on the suite.

So: tools/tests/test_suite_hygiene.py walks every test_*.py by AST (no imports,
so it works with or without any project venv) and asserts three things.

  * No file collects zero tests. A test_*.py with no collectable function is
    green and hollow.
  * No test requests a fixture nothing declares. `def test_x(suite)` makes
    pytest error on a missing fixture, and twelve such errors read as a config
    nit for months while twelve real audio cases went unrun.
  * No module-level sys.exit(). It fires during COLLECTION and takes the whole
    directory down instead of failing one test — which is how `pytest
    tools/tests/` once ran zero tests while 273 passed individually.

It also guards itself: if the glob ever stops finding files, every rule above
passes vacuously, so the file count has a floor of its own.

Verified by mutation, three planted defects, one per rule — a hollow file, a
`def test_thing(suite)`, and a trailing sys.exit(0). Each is caught by name and
the tree comes back clean.

Also splits tools/tests/test_setlist.py, whose 22 checks reported as one
aggregate assertion that named the file and not the fault. check() now records
each outcome and pytest replays them one named test per check; the aggregate
stays as the belt to those braces, and the file still runs as a script.

Every count is pinned, because the failure being guarded against is not a wrong
answer but an empty one, and an empty one is invisible without a number to
compare against. That guard earned itself immediately: I had counted 28 checks
by eye and the truth was 22.