pgsql: Add isolation test for INSERT ON CONFLICT speculative insertion

Add isolation test for INSERT ON CONFLICT speculative insertion failure.

This path previously was not reliably covered. There was some
heuristic coverage via insert-conflict-toast.spec, but that test is
not deterministic, and only tested for a somewhat specific bug.

Backpatch, as this is a complicated and otherwise untested code
path. Unfortunately 9.5 cannot handle two waiting sessions, and thus
cannot execute this test.

Triggered by a conversion with Melanie Plageman.

Author: Andres Freund
Discussion: https://postgr.es/m/CAAKRu_a7hbyrk=wveHYhr4LbcRnRCG=yPUVoQYB9YO1CdUBE9Q@mail.gmail.com
Backpatch: 9.5-

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0491a51fdd4e87fd5ab42ae42dfb9931cac681f6

Modified Files
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.../expected/insert-conflict-specconflict.out | 179 +++++++++++++++++++++
src/test/isolation/isolation_schedule | 1 +
.../specs/insert-conflict-specconflict.spec | 149 +++++++++++++++++
3 files changed, 329 insertions(+)