pgsql: Enforce our convention about max number of parallel regression t

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ef73a8162a5fe9c4b2f895bf9fb660f1aabc796c

Modified Files
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src/test/regress/GNUmakefile | 2 +-
src/test/regress/pg_regress.c | 25 ++++++++++++++++++++-----
src/tools/msvc/vcregress.pl | 2 ++
3 files changed, 23 insertions(+), 6 deletions(-)