t7300-clean: require POSIXPERM for chmod 0 test

A test case introduced by 91479b9c (t7300: add tests to document
behavior of clean and nested git) uses 'chmod 0' to verify that a
subdirectory that has an unreadable .git file is not removed. This can
work only when the system pays attention to the permissions set with
'chmod'. Therefore, set the POSIXPERM prerequisite on the test case.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7300-clean.sh

@@ -499,7 +499,7 @@ test_expect_success 'should not clean submodules' '
 	test_path_is_missing to_clean
 '
 
-test_expect_success 'should avoid cleaning possible submodules' '
+test_expect_success POSIXPERM 'should avoid cleaning possible submodules' '
 	rm -fr to_clean possible_sub1 &&
 	mkdir to_clean possible_sub1 &&
 	test_when_finished "rm -rf possible_sub*" &&