test.kt

@RunWith(MockitoJUnitRunner::class)
class SampleUnitTest {
    lateinit var syncUseCase: SyncUseCaseImpl
    lateinit var mdr: MusicDataRepository
    lateinit var plr: TestPlayListRepositoryImpl
    @Before
    fun createUseCase() {
        MockitoAnnotations.initMocks(this)
        mdr = Mockito.mock(MusicDataRepository::class.java)
        plr = TestPlayListRepositoryImpl()
        runBlocking {
            Mockito.`when`(mdr.getMusicData()).thenReturn(MusicData(ArrayList(), arrayListOf(
                    PlayList(0, "pl1", ""),
                    PlayList(1, "pl2", ""),
                    PlayList(2, "pl3", ""),
                    PlayList(3, "pl4", "")
            ), ArrayList()))
        }
        syncUseCase = SyncUseCaseImpl(mdr, plr)
    }

    @Test
    fun mdrTest() {
        plr.playLists = arrayListOf(
                PlayList(0, "pl1", ""),
                PlayList(1, "pl2", "")
        )
        runBlocking {
            syncUseCase.syncMusicData()
            assert(plr.getAll().size == 4)
        }
    }

    class TestPlayListRepositoryImpl : PlayListRepository {
        var playLists = ArrayList<PlayList>()
        override suspend fun getById(id: Long): PlayList? {
            TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
        }

        override suspend fun insert(pl: PlayList) {
            TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
        }

        override suspend fun insertAll(playLists: List<PlayList>) {
            this.playLists.addAll(playLists)
        }

        override suspend fun getAll(): List<PlayList> {
            return playLists
        }

    }
}

usecase.kt

override suspend fun syncMusicData() {
        val data = musicDataRepository.getMusicData()
        val playLists = playListRepository.getAll()
        val playListsToInsert = ArrayList<PlayList>()
        for (playList in data.playLists) {
            if (playLists.none { it.id == playList.id }) {
                playListsToInsert.add(playList)
            }
            if (playLists.contains(playList)) {
                continue
            }
        }
        System.out.println("lists to add = $playListsToInsert")
        playListRepository.insertAll(playListsToInsert)
    }

Очень много кода, а тест один и ассерт тоже один