tests: disable some chgrp tests when part of the root group

Some tests failed when run using Docker because they assumed the
user would never be root.  This is more of a band-aid solution.
An actual fix would be to test see if something like these tests
were to succeed when the user is root.

tests/tests.rs

@@ -4,6 +4,9 @@ mod common;
 #[macro_use]
 extern crate lazy_static;
 
+#[cfg(unix)]
+extern crate rust_users;
+
 // For conditional compilation
 macro_rules! unix_only {
     ($($fea:expr, $m:ident);+) => {