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/t7501-commit-basic-functionality.sh

@@ -21,7 +21,7 @@ test_expect_success 'initial status' '
 	echo bongo bongo >file &&
 	git add file &&
 	git status >actual &&
-	test_i18ngrep "No commits yet" actual
+	test_grep "No commits yet" actual
 '
 
 test_expect_success 'fail initial amend' '
@@ -141,7 +141,7 @@ test_expect_success 'template "emptyness" check does not kick in with -F' '
 test_expect_success 'template "emptyness" check' '
 	git checkout HEAD file && echo >>file && git add file &&
 	test_must_fail git commit -t file 2>err &&
-	test_i18ngrep "did not edit" err
+	test_grep "did not edit" err
 '
 
 test_expect_success 'setup: commit message from file' '
@@ -671,7 +671,7 @@ test_expect_success 'commit a file whose name is a dash' '
 	git add ./- &&
 	test_tick &&
 	git commit -m "add dash" >output </dev/null &&
-	test_i18ngrep " changed, 5 insertions" output
+	test_grep " changed, 5 insertions" output
 '
 
 test_expect_success '--only works on to-be-born branch' '