tests: Disable tests that require being non-root

Set the environment variable USER=root which is required by
brew tests --only=utils/user

Fix these tests:
rspec ./test/diagnostic_checks_spec.rb:26 # Homebrew::Diagnostic::Checks #check_access_directories
rspec ./test/pathname_spec.rb:120 # Pathname#ensure_writable makes a file writable and restores permissions
rspec ./test/utils/user_spec.rb:22 # User#gui? when the current user is in a console session gui? should equal true
rspec ./test/utils/user_spec.rb:6 # User should eq nil