Extend configure_test_server_for_ssl to add extensions

Extend configure_test_server_for_ssl to add extensions

In order to be able to test extensions with SSL connections, allow
configure_test_server_for_ssl to create any extensions passed as
an array. Each extension is created in all the test databases.

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