Simplify new test suite handling of passwordcheck

Simplify new test suite handling of passwordcheck

This changes the use of a custom configuration file to enforce the value
of preload_shared_libraries to simply load the library during the tests.
This removes the restriction of running installcheck on the tests, and
simplifies its makefile contrary to what has been introduced in af7211e.

Author: Michael Paquier <michael.paquier@gmail.com>