Fix tests under GETTEXT_POISON on parseopt

Use the i18n-specific test functions in test scripts for parseopt tests.
This issue was was introduced in v1.7.10.1-488-g54e6d:

    54e6d i18n: parseopt: lookup help and argument translations when showing usage

and been broken under GETTEXT_POISON=YesPlease since.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t2006-checkout-index-basic.sh b/t/t2006-checkout-index-basic.sh
index b855983..57cbdfe 100755
--- a/t/t2006-checkout-index-basic.sh
+++ b/t/t2006-checkout-index-basic.sh
@@ -7,7 +7,7 @@
 
 test_expect_success 'checkout-index --gobbledegook' '
 	test_expect_code 129 git checkout-index --gobbledegook 2>err &&
-	grep "[Uu]sage" err
+	test_i18ngrep "[Uu]sage" err
 '
 
 test_expect_success 'checkout-index -h in broken repository' '
@@ -18,7 +18,7 @@
 		>.git/index &&
 		test_expect_code 129 git checkout-index -h >usage 2>&1
 	) &&
-	grep "[Uu]sage" broken/usage
+	test_i18ngrep "[Uu]sage" broken/usage
 '
 
 test_done