pgsql: Add isolation tests for snapshot behavior in ri_triggers.c

Add isolation tests for snapshot behavior in ri_triggers.c

They are to check the behavior of RI_FKey_check() and
ri_Check_Pk_Match().  A test case whereby RI_FKey_check() queries a
partitioned PK table under REPEATABLE READ isolation produces wrong
output due to a bug of the partition-descriptor logic and that is noted
as such in the comment in the test.  A subsequent commit will fix the
bug and replace the buggy output by the correct one.

Author: Amit Langote <amitlangote09@gmail.com>
Discussion: https://postgr.es/m/1627848.1636676261@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/00cb86e75d6dd13a8bc07f79c2cdbe20d96c2d92

Modified Files
--------------
src/test/isolation/expected/fk-snapshot.out | 124 ++++++++++++++++++++++++++++
src/test/isolation/isolation_schedule       |   1 +
src/test/isolation/specs/fk-snapshot.spec   |  61 ++++++++++++++
3 files changed, 186 insertions(+)