t5510: skip tests under GETTEXT_POISON build

Skip tests when running under GETTEXT_POISON build and run them with
C_LOCALE_OUTPUT prerequisite.

These tests are irrelevant under GETTEXT_POISON because they test text
output alignment which GETTEXT_POISON turns useless.

Signed-off-by: Vasco Almeida <vascomalmeida@sapo.pt>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5510-fetch.sh

@@ -688,7 +688,7 @@ test_expect_success 'fetching with auto-gc does not lock up' '
 	)
 '
 
-test_expect_success 'fetch aligned output' '
+test_expect_success C_LOCALE_OUTPUT 'fetch aligned output' '
 	git clone . full-output &&
 	test_commit looooooooooooong-tag &&
 	(
@@ -703,7 +703,7 @@ test_expect_success 'fetch aligned output' '
 	test_cmp expect actual
 '
 
-test_expect_success 'fetch compact output' '
+test_expect_success C_LOCALE_OUTPUT 'fetch compact output' '
 	git clone . compact &&
 	test_commit extraaa &&
 	(