Add regular expression testing for user name mapping in the peer authentication TAP test

Hi hackers,

while working on [1], I thought it could also be useful to add regular 
expression testing for user name mapping in the peer authentication TAP 
test.

This kind of test already exists in kerberos/t/001_auth.pl but the 
proposed one in the peer authentication testing would probably be more 
widely tested.

Please find attached a patch proposal to do so.

[1]: 
https://www.postgresql.org/message-id/4f55303e-62c1-1072-61db-fbfb30bd66c8%40gmail.com

Looking forward to your feedback,

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com