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.

Cargo.toml

@@ -233,6 +233,7 @@ regex="*"
 rand="*"
 tempdir="*"
 unindent="*"
+lazy_static = "*"
 
 [[bin]]
 name = "uutils"