tests/chmod: don't make assumptions about umask

`test_chmod_many_options` relied on user's umask not denying read access
for anyone. 022, which is the default umask for many, indeed allows read
access for everyone. I'm using 027, which disallows read for everyone
but owner and group. This made tests fail.

Now tests set and reset umask, ensuring checks are run in a reliable,
predictable environment.

tests/test_chmod.rs

@@ -117,10 +117,16 @@ fn test_chmod_ugo_copy() {
 
 #[test]
 fn test_chmod_many_options() {
+    let original_umask = unsafe {
+        umask(0)
+    };
     let tests = vec!{
         TestCase{args: vec!{"-r,a+w", TEST_FILE}, before: 0o444, after: 0o222},
     };
     run_tests(tests);
+    unsafe {
+        umask(original_umask);
+    }
 }
 
 #[test]