t/lib-gpg: refactor `sanitize_pgp()` to not depend on Perl

t/lib-gpg: refactor `sanitize_pgp()` to not depend on Perl

The `sanitize_pgp()` test helper uses Perl to strip PGP signatures from
stdin. Refactor it to instead use sed(1) so that we drop the
PERL_TEST_HELPERS prerequisite in users of this library.

Note that we have to add PERL_TEST_HELPERS to a subset of tests in t6300
now that the test suite doesn't bail out early anymore in case the
prerequisite isn't set.

Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>