Add isolation tests for the combination of EPQ and triggers.

Add isolation tests for the combination of EPQ and triggers.

As evidenced by bug #16036 this area is woefully under-tested. Add
fairly extensive tests for the combination.

Backpatch back to 9.6 - before that isolationtester was not capable
enough. While we don't backpatch tests all the time, future fixes to
trigger.c would potentially look different enough in 12+ from the
earlier branches that introducing bugs during backpatching is more
likely than normal. Also, it's just a crucial and undertested area of
the code.

Author: Andres Freund
Discussion: https://postgr.es/m/16036-28184c90d952fb7f@postgresql.org
Backpatch: 9.6-, the earliest these tests work