t5000: use check_mtime()

fd2da4b1ea (archive: add --mtime, 2023-02-18) added a helper function
for checking the file modification time of an extracted entry.  Use it
for the older mtime test as well to shorten the code and piggyback on
the archive extraction done to validate file contents.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5000-tar-tree.sh

@@ -185,6 +185,7 @@ test_expect_success 'git archive' '
 '
 
 check_tar b
+check_mtime b a/a 1117231200
 
 test_expect_success 'git archive --mtime' '
 	git archive --mtime=2002-02-02T02:02:02-0200 HEAD >with_mtime.tar
@@ -257,14 +258,6 @@ test_expect_success 'git archive --remote with configured remote' '
 	test_cmp_bin b.tar b5-nick.tar
 '
 
-test_expect_success 'validate file modification time' '
-	mkdir extract &&
-	"$TAR" xf b.tar -C extract a/a &&
-	test-tool chmtime --get extract/a/a >b.mtime &&
-	echo "1117231200" >expected.mtime &&
-	test_cmp expected.mtime b.mtime
-'
-
 test_expect_success 'git get-tar-commit-id' '
 	git get-tar-commit-id <b.tar >actual &&
 	git rev-parse HEAD >expect &&