Fix test for cleanup failure in t7300 on Windows

Keep the file open to: the OS does not allow removal of open files.
The saner systems just have a saner permission model and chmod 0
is enough for the test.
Signed-off-by: NAlex Riesen <raa.lkml@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t7300-clean.sh

@@ -320,8 +320,9 @@ test_expect_success 'removal failure' '
 
 	mkdir foo &&
 	touch foo/bar &&
+	exec <foo/bar &&
 	chmod 0 foo &&
-	! git clean -f -d
+	test_must_fail git clean -f -d
 
 '
 chmod 755 foo