t9001: fix broken "invoke hook" test

This test has been dysfunctional since it was added by 6489660b4b
(send-email: support validate hook, 2017-05-12), however, the problem
went unnoticed due to a broken &&-chain late in the test.

The test wants to verify that a non-zero exit code from the
'sendemail-validate' hook causes git-send-email to abort with a
particular error message. A command which is expected to fail should be
run with 'test_must_fail', however, the test neglects to do so.

Fix this problem, as well as the broken &&-chain behind which the
problem hid.

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

t/t9001-send-email.sh

@@ -1966,11 +1966,11 @@ test_expect_success $PREREQ 'invoke hook' '
 
 		# Verify error message when a patch is rejected by the hook
 		sed -e "s/add master/x/" ../0001-add-master.patch >../another.patch &&
-		git send-email \
+		test_must_fail git send-email \
 			--from="Example <nobody@example.com>" \
 			--to=nobody@example.com \
 			--smtp-server="$(pwd)/../fake.sendmail" \
-			../another.patch 2>err
+			../another.patch 2>err &&
 		test_i18ngrep "rejected by sendemail-validate hook" err
 	)
 '