t/lib-gpg: sanity-check that we can actually sign

t/lib-gpg: sanity-check that we can actually sign

Some older versions of gpg (reportedly v1.2.6 from RHEL4) cannot
import the keyrings found in our test suite, and thus cannot even
make a signature.  The previous change works it around, but we
cannot anticipate breakages update to GPG would cause in the future.

Do a test-sign before declaring the GPG prerequisite fulfilled
to future-proof our tests.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>