t5551: use 'test_i18ngrep' to check translated output

The two tests 'invalid Content-Type rejected' and 'server-side error
detected' in 't5551-http-fetch-smart.sh' use "plain" 'grep' to check
that 'git clone' failed with the expected error message, but the
messages they are checking are translated, and, consequently, these
tests fail when the test script is run with GIT_TEST_GETTEXT_POISON
enabled.

Use 'test_i18ngrep' instead.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5551-http-fetch-smart.sh

diff --git a/t/t5551-http-fetch-smart.sh b/t/t5551-http-fetch-smart.sh
index ac74626..cb11f13 100755
--- a/t/t5551-http-fetch-smart.sh
+++ b/t/t5551-http-fetch-smart.sh
@@ -199,7 +199,7 @@
 
 test_expect_success 'invalid Content-Type rejected' '
 	test_must_fail git clone $HTTPD_URL/broken_smart/repo.git 2>actual &&
-	grep "not valid:" actual
+	test_i18ngrep "not valid:" actual
 '
 
 test_expect_success 'create namespaced refs' '
@@ -466,7 +466,7 @@
 
 test_expect_success 'server-side error detected' '
 	test_must_fail git clone $HTTPD_URL/error_smart/repo.git 2>actual &&
-	grep "server-side error" actual
+	test_i18ngrep "server-side error" actual
 '
 
 test_done