Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Use TAP_TESTS_PERL to register the legacy_test in bin/pax.

Redo r260586 by using the new functionality in tap.test.mk to transparently
support perl-based test programs.

As a side-effect, we get rid of an explicit path to /usr/bin/perl by
replacing it with /usr/local/bin/perl (or as defined in tap.test.mk).

This also fixes the name of the legacy_test source file because this should
have always been legacy_test.pl and not legacy_test.sh. My mistake when
originally moving the code around without realizing that this was a perl
script.

MFC after: 5 days