Further improve stability of partition_prune regression test.

Commits 4ea03f3f et al arranged to filter out row counts in parallel
plans, because those are dependent on the number of workers actually
obtained.  Somehow I missed that the 'Rows Removed by Filter' counts
can also vary, so fix that too.  Per buildfarm.

This seems worth a last-minute patch because unreliable regression
tests are a serious pain in the rear for packagers.

Like the previous patch, back-patch to v11 where this test was
introduced.