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>

t/lib-gpg.sh

@@ -36,6 +36,8 @@ else
 			"$TEST_DIRECTORY"/lib-gpg/keyring.gpg &&
 		gpg --homedir "${GNUPGHOME}" 2>/dev/null --import-ownertrust \
 			"$TEST_DIRECTORY"/lib-gpg/ownertrust &&
+		gpg --homedir "${GNUPGHOME}" </dev/null >/dev/null 2>&1 \
+			--sign -u committer@example.com &&
 		test_set_prereq GPG
 		;;
 	esac