Check for tabs in postgresql.conf.sample.

Check for tabs in postgresql.conf.sample.

The previous commit updated this file to use spaces instead of
tabs.  This commit adds a test to ensure that no new tabs are
added.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/aReNUKdMgKxLqmq7%40nathan