How does JUnit 6 support for Kotlin differ from JUnit 5?

// Kotlin JUnit 6 test class
import org.junit.jupiter.api.*
import org.junit.jupiter.api.Assertions.*

@TestInstance(TestInstance.Lifecycle.PER_CLASS)
class KotlinOrderTest {

    private lateinit var service: OrderService

    // No @JvmStatic needed with PER_CLASS lifecycle
    @BeforeAll
    suspend fun setUpAll() {
        service = OrderService()
        service.connect()   // suspend function -- works directly!
    }

    @BeforeEach
    suspend fun setUp() {
        service.reset()     // suspend function
    }

    @Test
    suspend fun `new order has pending status`() {
        val order = service.create("item-1", 2)  // suspend call
        assertEquals(Status.PENDING, order.status)
    }

    @Test
    suspend fun `invalid quantity throws`() {
        assertThrows<IllegalArgumentException> {
            service.create("item-1", -1)   // suspend call
        }
    }

    // JSpecify: smart cast works after assertNotNull
    @Test
    fun `smart cast after assertNotNull`() {
        val result: String? = service.findUser("user-1")
        assertNotNull(result)       // JUnit 6 exposes @NonNull contract
        println(result.length)      // No !! needed -- smart cast works
    }

    @AfterAll
    suspend fun tearDownAll() {
        service.disconnect()        // suspend function
    }
}