pgsql: Add subxid-overflow "isolation" test

Add subxid-overflow "isolation" test

This test covers a few lines of subxid-overflow-handling code in various
part of the backend, which are otherwise uncovered.

Author: Simon Riggs <simon.riggs@enterprisedb.com>
Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com>
Discussion: https://postgr.es/m/CANbhV-H8ov5+nCMBYQFKhO+UZJjrFgY_ORiMWr3RhS4+x44PzA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0e733278e346f19df93843bce9a81c8183df6e1d

Modified Files
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src/include/storage/proc.h                      |  2 +
src/test/isolation/expected/subxid-overflow.out | 82 +++++++++++++++++++++++++
src/test/isolation/isolation_schedule           |  1 +
src/test/isolation/specs/subxid-overflow.spec   | 79 ++++++++++++++++++++++++
4 files changed, 164 insertions(+)