@Test
public void onHalfOpen_adds_runnable_to_list_and_returns_self() {

    // given
    Runnable listener = mock(Runnable.class);
    // when
    CircuitBreaker<String> fluentReturn = cbSpy.onHalfOpen(listener);
    // then
    assertThat(cbSpy.onHalfOpenListeners).contains(listener);
    assertThat(fluentReturn).isSameAs(cbSpy);
}