[nmh-workers] test-scan passes for me, but is in XFAIL_TESTS

[Ken Hornstein]

In commit d9b33232b6, test-scan was added to XFAIL_TESTS.  But for me
it succeeds.

Looking closer ... I guess this is because MacOS X lacks a "/dev/full",
which I didn't even know was a thing.  And I guess this is all related to
dealing with Bug #50925.

Since this test doesn't fail on all platforms, is everyone ok with me
just commenting out this test for now and moving test-scan back to
TESTS?  I am trying to make it so the test suite passes successfully on
MacOS X after a long while of NOT passing, and this is the last remaining
problem :-)

--Ken