Add TAP tests for contrib/sslinfo

Add TAP tests for contrib/sslinfo

This adds rudimentary coverage of the sslinfo extension into the SSL
test harness.  The output is validated by comparing with pg_stat_ssl
to provide some level of test stability should the underlying certs
be slightly altered.  A new cert is added to provide an extension to
test against.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Reviewed-by: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Discussion: https://postgr.es/m/E23F9811-0C77-45DA-912F-D809AB140741@yesql.se