t4014: invoke format-patch with --stdout where intended

The test wrote something along the lines of 0001-foo.patch to output,
which of course never contained a signature.  Luckily the tested
behaviour is actually present.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4014-format-patch.sh

@@ -698,8 +698,8 @@ test_expect_success 'format-patch --no-signature supresses signatures' '
 	! grep "^-- \$" output
 '
 
-test_expect_failure 'format-patch --signature="" supresses signatures' '
-	git format-patch --signature="" -1 >output &&
+test_expect_success 'format-patch --signature="" supresses signatures' '
+	git format-patch --stdout --signature="" -1 >output &&
 	check_patch output &&
 	! grep "^-- \$" output
 '