1.  Create a new test class:

class MyTest {
}

2. Create a mocked instance of the RegistrationApi interface as the test-class property:

class MyTest {
    private val api = mock<RegistrationApi>()
}

3. Add a class property of the RegistrationFormController type:

class MyTest {
    private val api = mock<RegistrationApi>()
    private var registrationFormController = 
        RegistrationFormController(api = api)
}

4. Create the test method to check whether checkIfEmailCanBeRegistered() behaves correctly for an invalid email address occurrence: 

class MyTest {
    private val api = mock<RegistrationApi>()
    private lateinit var registrationFormController: RegistrationFormController

    @Before
    fun setup() {
        registrationFormController = RegistrationFormController(api = api)
    }

    @Test
    fun `email shouldn't be registered if it's not valid`() {
        // given
        assertNotNull(registrationFormController)
        whenever(api.isEmailAddressAvailable(anyString())) doReturn(true)
        // when
        registrationFormController.currentEmailAddress = "Hilary"
        // then
        assertFalse(registrationFormController.checkIfEmailCanBeRegistered())
    }
}