t7300: mark test with SANITY

The test runs `chmod 0` on a file to test a case where Git fails to
read it, but that would not work if it is run as root.

Reported-by: Jan Keromnes <janx@linux.com>
Fix-proposed-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7300-clean.sh

@@ -495,7 +495,7 @@ test_expect_success 'should not clean submodules' '
 	test_path_is_missing to_clean
 '
 
-test_expect_success POSIXPERM 'should avoid cleaning possible submodules' '
+test_expect_success POSIXPERM,SANITY 'should avoid cleaning possible submodules' '
 	rm -fr to_clean possible_sub1 &&
 	mkdir to_clean possible_sub1 &&
 	test_when_finished "rm -rf possible_sub*" &&