Add checks for regexes with user name map in test for peer authentication

Add checks for regexes with user name map in test for peer authentication

There is already some coverage for that in the kerberos test suite,
though it requires PG_TEST_EXTRA to be set as per its insecure nature.
This provides coverage in a default setup, as long as peer is supported
on the platform where its test is run.

Author: Bertrand Drouvot
Discussion: https://postgr.es/m/7f87ca27-e184-29da-15d6-8be4325ad02e@gmail.com