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.

Cargo.toml

@@ -252,6 +252,7 @@ unindent = "0.1.0"
 lazy_static = "0.2.2"
 
 [target.'cfg(unix)'.dev-dependencies]
+rust-users = { git = "https://github.com/Arcterus/rust-users" }
 unix_socket = "0.5.0"
 
 [[bin]]