This stubs the find method on `Group` to always return `group` (whatever that is).

E.g. if I now write `Group.find(5)` it will return `group`, same for `Group.find(1337)` or `Group.find`. 

About stubs etc. in general check out [rspec-mocks](https://github.com/rspec/rspec-mocks). However be aware that the docs use the new syntax (to which we should update as well). E.g. they don't write `Group.stub(:method).and_return(value)` but `allow(Group).to receive(:method).and_return(value)`