Add authentication TAP test for peer authentication

Add authentication TAP test for peer authentication

This commit introduces an authentication test for the peer method, as of
a set of scenarios with and without a user name map.  The script is
automatically skipped if peer is not supported in the environment where
this test is run, checking this behavior by attempting a connection
first on a cluster up and running.

Author: Bertrand Drouvot
Discussion: https://postgr.es/m/aa60994b-1c66-ca7a-dab9-9a200dbac3d2@amazon.com