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/t2010-checkout-ambiguous.sh

@@ -62,8 +62,8 @@ test_expect_success 'disambiguate checking out from a tree-ish' '
 
 test_expect_success 'accurate error message with more than one ref' '
 	test_must_fail git checkout HEAD main -- 2>actual &&
-	test_i18ngrep 2 actual &&
-	test_i18ngrep "one reference expected, 2 given" actual
+	test_grep 2 actual &&
+	test_grep "one reference expected, 2 given" actual
 '
 
 test_done