Add tests for regex replacement with \1 in pg_ident.conf to 0003_peer.pl

Add tests for regex replacement with \1 in pg_ident.conf to 0003_peer.pl

Regexp replacement with \1 in pg_ident.conf is tested in one check of
the kerberos test suite, still it requires a dependency on
--with-gssapi to be triggered.  This commit adds to the test suite of
peer authentication two tests to check the replacement of \1 in a
pg-username, coupled with a system-username regexp:
- With a subexpression in system-username, similarly to the kerberos
test suite.
- Without a subexpression in system-username, checking for a failure.
This had no coverage until now, and the error pattern is checked in the
server logs.

Author: Jelte Fennema
Discussion: https://postgr.es/m/CAGECzQRNow4MwkBjgPxywXdJU_K3a9+Pm78JB7De3yQwwkTDew@mail.gmail.com