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 
Discussion: https://postgr.es/m/1627848.1636676261@sss.pgh.pa.us