Improve stability of partition_prune regression test.

This test already knew that, to get stable test output, it had to hide
"loops" counts in EXPLAIN ANALYZE results.  But that's not nearly enough:
if we get a smaller number of workers than we planned for, then the
"Workers Launched" number will change, and so will all the rows and loops
counts up to the Gather node.  This has resulted in repeated failures in
the buildfarm, so adjust the test to filter out all these counts.

(Really, we wouldn't bother with EXPLAIN ANALYZE at all here, except
that currently the only way to verify that executor-time pruning has
happened is to look for '(never executed)' annotations.  Those are
stable and needn't be filtered out.)

Back-patch to v11 where the test was introduced.

Discussion: https://postgr.es/m/11952.1569536725@sss.pgh.pa.us