Tighten the per-operator testing done in brin regression test.

Verify that the number of matches is exactly what it should be, not just
that it not be zero.  This should help us detect any environment-dependent
issues.

Also, verify that we're getting the expected type of scan plan (either
bitmap or seqscan as appropriate).  Right now, this is failing on the
cidrcol test cases, as shown in the output file.  I'll look into that
in a bit, but it seems good to commit this as-is temporarily to verify
that it behaves as expected on the buildfarm.