test-lib-functions.sh: fix test_grep fail message wording

In the recent commit 2e87fca189 (test framework: further deprecate
test_i18ngrep, 2023-10-31), the test_i18ngrep function was
deprecated, and all the callers were updated to call the test_grep
function instead.  But test_grep inherited an error message that
still refers to test_i18ngrep by mistake.  Correct it so that a
broken call to the test_grep will identify itself as such.

Signed-off-by: Shreyansh Paliwal <shreyanshpaliwalcmsmn@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>