tests/chmod: protect umask with a mutex

`test_chmod_ugoa` and `test_chmod_many_options` both change umask, which
is global state. Since tests run concurrently, this might lead to
a situation where one of the tests changes umask to a value that screws
another test's checks. To prevent this, we introduce a mutex that should
be held by any test that changes umask.

Unfortunately, there's no way to hide umask behind this mutex and
enforce its usage: programmers will have to maintain the discipline
themselves.

tests/tests.rs

@@ -1,6 +1,9 @@
 #[macro_use]
 mod common;
 
+#[macro_use]
+extern crate lazy_static;
+
 // For conditional compilation
 macro_rules! unix_only {
     ($($fea:expr, $m:ident);+) => {