t7508: fix bogus mtime verification

The current `grep`-approach in "--no-optional-locks prevents index
update" may fail e.g. for `out` file contents "1234567890999" [1].
Fix this by using test-lib's new mtime-verification API.

[1] https://lore.kernel.org/git/xmqqczl5hpaq.fsf@gitster.g/T/#u

Signed-off-by: Marc Strapetz <marc.strapetz@syntevo.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7508-status.sh

@@ -1647,13 +1647,11 @@ test_expect_success '"Initial commit" should not be noted in commit template' '
 '
 
 test_expect_success '--no-optional-locks prevents index update' '
-	test-tool chmtime =1234567890 .git/index &&
+	test_set_magic_mtime .git/index &&
 	git --no-optional-locks status &&
-	test-tool chmtime --get .git/index >out &&
-	grep ^1234567890 out &&
+	test_is_magic_mtime .git/index &&
 	git status &&
-	test-tool chmtime --get .git/index >out &&
-	! grep ^1234567890 out
+	! test_is_magic_mtime .git/index
 '
 
 test_done