t6302: skip only signed tags rather than all tests when GPG is missing

t6302: skip only signed tags rather than all tests when GPG is missing

The primary purpose of these tests is to check filtering, sorting, and
formatting behavior of git-for-each-ref, so it is unfortunate that the
entire test script is skipped when GPG is not present. Rather than
skipping all tests, let's instead just skip testing against signed tags
when GPG is missing.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>