The partition_join test was added to a parallel group that was already
at the maximum of 20 concurrent tests.  The hash_func test wasn't
added to serial_schedule at all.  The identity and partition_join tests
were added to serial_schedule with the aid of a dartboard, rather than
maintaining consistency with parallel_schedule.

There are proposals afoot to make these sorts of errors harder to make,
but in the meantime let's fix the ones already in place.

Discussion: https://postgr.es/m/a37e9c57-22d4-1b82-1270-4501cd2e984e@2ndquadrant.com