You can test methods that return void. Their purpose is change the state of an object, therefore you can test whether the state has changed as expected. It means that you must be able to somehow observe the state from your test method.
Usually the object should expose the state by a method. You'll call the method and check the response.
If there is nothing public that you could check, but there is something protected, you could create a child of the class (in your test project) and expose the state there.
If all you have is private variable, you can't read it from the test class. But you could observe the behaviour influenced by the changed state and verify whether changing the state had the expected effect.
In general, the point is testing the behavior, not that the assignment operator works.
By the way, you method looks strange. If you meant to implement a normal parm method, you should return a value. If you want to have just a setter, remove the default parameter, because calling the method without the parameter makes no sense.
