matches_the_pg_string isn't unit tested

[malsyned]

If matches_the_pg_string_test() was rewritten to simply "return true", the test suite would still pass and many of the tests in the self-test suite could be in error. While it is easy to catch errors in this test while adding new self-tests (greens would show up when the developer is expecting reds), refactoring in pfstest-matchers.c could result in this function breaking silently until the problem is discovered while adding new self-tests, if ever.