PERL5LIB and taint

This is a fix for [rt.cpan.org 84377](https://rt.cpan.org/Ticket/Display.html?id=84377).  It ensures PERL5LIB is always propagated into a tests' `@INC` even if the test has taint mode on.  This makes running tests much more consistent and makes it possible to run taint mode tests in a local::lib or Perl home dir library environment.

Test::Harness already has code to ensure this.  TAP::Harness (and thus prove) did not.  It's possible the Test::Harness code is now redundant.