Test suite --only doesn't work for hard_hole_fits

I tried to test hard_hole_fits all by itself

hadrian/build test  --skip=_build/stage0/** --skip=_build/stage1/**  --docs=no-haddocks --only=hard_hole_fits

But alas it is a no-op:

...
SUMMARY for test run started at Wed Sep 13 13:55:02 2023 
0:00:00.100124 spent to go through
       1 total tests, which gave rise to
       3 test cases, of which
       3 were skipped
       0 had missing libraries

       0 expected passes
       0 expected failures

       0 caused framework failures
       0 caused framework warnings
       0 unexpected passes
       0 unexpected failures
       0 unexpected stat failures
       0 fragile tests

Can anyone explain why/help? @bgamari @mpickering @monoidal?

It's a pain because I want to add --test-accept to accept the new output, but I can't. I can't even do it by hand because I don't know exactly what flags are passed to GHC.