Replace flaky CIC/RI isolation tests with a TAP test

Replace flaky CIC/RI isolation tests with a TAP test

The isolation tests for INSERT ON CONFLICT behavior during CREATE INDEX
CONCURRENTLY and REINDEX CONCURRENTLY (added by bc32a12e0db2,
2bc7e886fc1b, and 90eae926abbb) were disabled in 77038d6d0b49 due to
persistent CI flakiness, after several attempts at stabilization.

This commit removes them and introduces a TAP test in test_misc module
(010_index_concurrently_upsert.pl) that covers the same scenarios.  This
new test should hopefully be more stable while providing assurance that
the fixes in all those commits (plus 81f72115cf18) continue to work.

Author: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Reported-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://postgr.es/m/ccssrhafzbp3a3beju3ptyc56a7gbfimj4vwkbokoldofckrc7@bso37rxskjtf
Discussion: https://postgr.es/m/CANtu0ogv+6wqRzPK241jik4U95s1pW3MCZ3rX5ZqbFdUysz7Qw@mail.gmail.com
Discussion: https://postgr.es/m/202512112014.icpomgc37zx4@alvherre.pgsql