Test whether call is done in parallel - kotlin-coroutines

Question

Code:
object ParallelCall {
suspend fun <T, V> Iterable<T>.runInParallel(method: suspend (T) -> V): Iterable<V> {
//If List is empty then return empty List.
//Else, call the given method for each element in the list in parallel and return the list.
}
}
I have below unit test:
import kotlinx.coroutines.runBlocking
import org.junit.jupiter.api.Test
import kotlinx.coroutines.delay
import org.mockito.Mockito.spy
import org.mockito.Mockito.verify
import org.mockito.Mockito.never
import org.mockito.Mockito.times
import kotlin.test.assertTrue
import kotlin.test.assertEquals
open class ParallelTest {
#Test
fun `test runInParallel with empty list`() {
val spy = spy(this)
val emptyList = emptyList<Int>()
val result = runBlocking {
emptyList.runInParallel {
delay(100)
appendTextToInput()
}
}
assertTrue(result.toList().isEmpty())
verify(spy, never()).appendTextToInput()
}
#Test
fun `test runInParallel is parallel when input is non-empty list`() {
val spy = spy(this)
val inputList = listOf(1, 2, 3, 4, 5)
val startTime = System.currentTimeMillis()
val result = runBlocking {
inputList.runInParallel {
delay(100)
appendTextToInput()
}
}
val endTime = System.currentTimeMillis()
val totalTime = endTime - startTime
assertEquals(inputList.map { appendTextToInput() }, result.toList())
//If calls are executed in parallel the time taken should be < 500 milliseconds (5 * 100 milliseconds)
assertTrue(totalTime < 500)
verify(spy, times(inputList.size)).appendTextToInput()
}
private fun appendTextToInput(): String {
return "asynchronous call"
}
}
The unit test works when running each method separately. If I run the whole test class then one of the test case fails. It looks like the spy is shared among the test cases which is causing the issue in verify because object is a singleton class in Kotlin. How can I fix this?