Use library functions to edit config in SSL tests
The SSL tests were editing the postgres configuration by directly
reading and writing the files rather than using append_conf() from
the testcode library.
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/
01F4684C-8C98-4BBE-AB83-
AC8D7C746AF8@yesql.se