Detect unused steps in isolation specs and do some cleanup

Michael Paquier <michael@paquier.xyz>

Commit: 96f3661e4540db1f45709c5f098aeecc6e9df271
Author: Michael Paquier <michael@paquier.xyz>
Date: 2021-06-17T02:57:21Z
Releases: 12.8
Detect unused steps in isolation specs and do some cleanup

This is useful for developers to find out if an isolation spec is
over-engineered or if it needs more work by warning at the end of a
test run if a step is not used, generating a failure with extra diffs.

While on it, clean up all the specs which include steps not used in any
permutations to simplify them.

This is a backpatch of 989d23b and 06fdc4e, as it is becoming useful to
make all the branches consistent for an upcoming patch that will improve
the output generated by isolationtester.

Author: Michael Paquier
Reviewed-by: Asim Praveen, Melanie Plageman
Discussion: https://postgr.es/m/20190819080820.GG18166@paquier.xyz
Discussion: https://postgr.es/m/794820.1623872009@sss.pgh.pa.us
Backpatch-through: 9.6

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