pgsql: Add test for temporal referential integrity

Add test for temporal referential integrity

This commit adds an isolation test showing that temporal foreign keys do
not permit referential integrity violations under concurrency, like
fk-snapshot-2.  You can show that the test fails by passing false for
detectNewRows to ri_PerformCheck in ri_restrict.

Author: Paul Jungwirth <pj@illuminatedcomputing.com>
Reviewed-by: Rustam ALLAKOV <rustamallakov@gmail.com>
Reviewed-by: Dean Rasheed <dean.a.rasheed@gmail.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/CA+renyUp=xja80rBaB6NpY3RRdi750y046x28bo_xg29zKY72Q@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e8cec3d1791c140398454aa561cf51659dd8243d

Modified Files
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src/test/isolation/expected/fk-snapshot-3.out | 213 ++++++++++++++++++++++++++
src/test/isolation/isolation_schedule         |   1 +
src/test/isolation/specs/fk-snapshot-3.spec   |  82 ++++++++++
3 files changed, 296 insertions(+)