Test LiveData and Coroutines using MockK

I have this view model:

class MyViewModel(private val myUseCase: MyUseCase) : ViewModel() {

    val stateLiveData = MutableLiveData(State.IDLE)

    fun onButtonPressed() {
        viewModelScope.launch {
            stateLiveData.value = State.LOADING
            myUseCase.loadStuff() // Suspend
            stateLiveData.value = State.IDLE
        }
    }
}

I'd like to write a test that checks whether the state is really LOADING while myUseCase.loadStuff() is running. I'm using MockK for that. Here's the test class:

@ExperimentalCoroutinesApi
class MyViewModelTest {

    @get:Rule
    val rule = InstantTaskExecutorRule()

    private lateinit var myUseCase: MyUseCase
    private lateinit var myViewModel: MyViewModel

    @Before
    fun setup() {
        myUseCase = mockkClass(MyUseCase::class)
        myViewModel = MyViewModel(myUseCase)
    }

    @Test
    fun `button click should put screen into loading state`() = runBlockingTest {
        coEvery { myUseCase.loadStuff() } coAnswers  { delay(2000) }
        myViewModel.onButtonPressed()
        advanceTimeBy(1000)
        val state = myViewModel.stateLiveData.value
        assertEquals(State.LOADING, state)
    }
}

It fails:

java.lang.AssertionError: 
Expected :LOADING
Actual   :IDLE

How can I fix this?