t/lib-gpg: require GPGSSH for GPGSSH_VERIFYTIME prereq

The GPGSSH_VERIFYTIME prequeq makes use of "${GNUPGHOME}" but does not create it. Require GPGSSH which creates the "${GNUPGHOME}" directory. Additionally, it makes sense to require GPGSSH in GPGSSH_VERIFYTIME because the latter builds on the former. If we can't use GPGSSH, there's little point in checking whether GPGSSH_VERIFYTIME is usable. Suggested-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Todd Zullinger <tmz@pobox.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>