ErrorHandler.kt

class ErrorHandler(private val logger: Logger) {
  
  fun handleError() {
    GlobalScope.launch(Dispatchers.IO) {
      heavyOperation()
      logger.logErrorEvent()
    }
    
}

UnitTest.kt

@Test
fun test() = runBlocking<Unit> {
    val testee = ErrorHandler(logger)
    testee.handleError()
    verify(logger).logErrorEvent()
}

Example of a failing unit test.

This test will fail because the unit test will complete before the coroutine will have had a change to launch and execute.