Enforce our convention about max number of parallel regression tests.

We have a very old rule that parallel_schedule should have no more
than twenty tests in any one parallel group, so as to provide a
bound on the number of concurrently running processes needed to
pass the tests.  But people keep forgetting the rule, so let's add
a few lines of code to check it.

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

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