tests: teach callers of test_i18ngrep to use test_grep

They are equivalents and the former still exists, so as long as the
only change this commit makes are to rewrite test_i18ngrep to
test_grep, there won't be any new bug, even if there still are
callers of test_i18ngrep remaining in the tree, or when merged to
other topics that add new uses of test_i18ngrep.

This patch was produced more or less with

    git grep -l -e 'test_i18ngrep ' 't/t[0-9][0-9][0-9][0-9]-*.sh' |
    xargs perl -p -i -e 's/test_i18ngrep /test_grep /'

and a good way to sanity check the result yourself is to run the
above in a checkout of c4603c1c (test framework: further deprecate
test_i18ngrep, 2023-10-31) and compare the resulting working tree
contents with the result of applying this patch to the same commit.
You'll see that test_i18ngrep in a few t/lib-*.sh files corrected,
in addition to the manual reproduction.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1404-update-ref-errors.sh

@@ -29,7 +29,7 @@ test_update_rejected () {
 	fi &&
 	printf "create $prefix/%s $C\n" $create >input &&
 	test_must_fail git update-ref --stdin <input 2>output.err &&
-	test_i18ngrep -F "$error" output.err &&
+	test_grep -F "$error" output.err &&
 	git for-each-ref $prefix >actual &&
 	test_cmp unchanged actual
 }
@@ -613,7 +613,7 @@ test_expect_success REFFILES 'delete fails cleanly if packed-refs file is locked
 	test_when_finished "rm -f .git/packed-refs.lock" &&
 	test_must_fail git update-ref -d $prefix/foo >out 2>err &&
 	git for-each-ref $prefix >actual &&
-	test_i18ngrep "Unable to create $SQ.*packed-refs.lock$SQ: " err &&
+	test_grep "Unable to create $SQ.*packed-refs.lock$SQ: " err &&
 	test_cmp unchanged actual
 '