pgsql: Test decoding of two-phase transactions during the build of a co

Test decoding of two-phase transactions during the build of a consistent snapshot.

Commit a271a1b50e added the capability to allow decoding at prepare time.
This adds an isolation testcase to test that decoding happens at commit
time when the consistent snapshot state is reached after prepare but
before commit prepared.

Author: Ajin Cherian
Reviewed-by: Amit Kapila
Discussion:
https://postgr.es/m/02DA5F5E-CECE-4D9C-8B4B-418077E2C010@postgrespro.ru
https://postgr.es/m/CAMGcDxeqEpWj3fTXwqhSwBdXd2RS9jzwWscO-XbeCfso6ts3+Q@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/5a3574d7b33480de51692962e6fd25d00c05718a

Modified Files
--------------
contrib/test_decoding/Makefile                     |  3 +-
.../test_decoding/expected/twophase_snapshot.out   | 41 +++++++++++++++++
contrib/test_decoding/specs/twophase_snapshot.spec | 53 ++++++++++++++++++++++
3 files changed, 96 insertions(+), 1 deletion(-)