Check the group membership using dscl on Mac in specs.

Fixes the intermittent spec issue in our CI: 

```
  1) Chef::Resource::Group group manage action when there is a group behaves like correct group management when append is not set should remove the existing users and add the new users to the group
     Failure/Error: user_exist_in_group?(member).should == true
       expected: true
            got: false (using ==)
     Shared Example Group: "correct group management" called from ./spec/functional/resource/group_spec.rb:374
     # ./spec/functional/resource/group_spec.rb:106:in `block in add_members_to_group'
     # ./spec/functional/resource/group_spec.rb:105:in `each'
     # ./spec/functional/resource/group_spec.rb:105:in `add_members_to_group'
     # ./spec/functional/resource/group_spec.rb:136:in `block (4 levels) in <top (required)>'
```

@opscode/client-engineers  I would like to backport this to 11-stable and get it into the 11.18 release so that we can fix this spec issue which prevents daily Chef DK releases. @adamedx any concerns?