Add test for postgresql.conf.sample line syntax

Add test for postgresql.conf.sample line syntax

All GUCs in postgresql.conf.sample should be set to the default
value and be commented out.  This syntax was however not tested
for, making omissions easy to miss.  Add a test which check all
lines for syntax.

Author: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://postgr.es/m/19727040-3EE4-4719-AF4F-2548544113D7@yesql.se