I don't believe mixins interfere with testing although you may have to adapt how you approach the tests a little. First off, you shouldn't put the behaviour directly into the SigninController object. Instead create a SigninController trait and have a companion object that extends that trait:

trait SigninControler extends Controller with ReturnUrlVerifier {
...
}
object SigninController extends SigninController

Then your tests all test the trait, not the object. You can instantiate an anonymous instance of the trait easily and you can override any behaviour you need to change in the super traits:

val controller =  new SigninController{
override def someStubbedMethod(){...}
}

The benefit of this approach is that your tests will be testing the actual behaviour of the classes/objects you are testing most of the time with almost no need for mocking/stubbing out methods. But if you do want to test specific implementation details you still have the flexibility to do so using the override sytax which is so concise in Scala it is usally as easy or easier than using Mockito.