Add postgres version variants for transaction tests

There are some subtle differences in the outputs of the transaction handler
tests, due to varying GUC defaults and error outputs between postgres versions.
Account for this by making use of variant comparison files:
https://www.postgresql.org/docs/current/regress-variant.html

This commit specifically makes use of the second mechanism, where:
- set_user_1.out corresponds to variations in PostgreSQL version 9.6
- set_user_2.out corresponds to variations in PostgreSQL version 9.4