The test coroutine scheduler isn't closed when a test ends

[dkhalanskyjb]

Describe the bug

The kotlinx-coroutines-test test framework may fail to run some code to completion if a test dispatcher was passed to the system under test.
The recommended approach is to pass a CoroutineScope to the system under test, which allows the test to know that some operations in the system under test may still be running. When only a dispatcher is passed, a new task may arrive there by the time runTest is no longer processing the tasks in the test scheduler.

Provide a Reproducer

import kotlinx.coroutines.test.*
import kotlinx.coroutines.*
import kotlin.time.Duration.Companion.milliseconds
import kotlin.concurrent.thread

suspend fun waitForSomethingAsynchronousToHappen() {
    val latch = CompletableDeferred<Unit>()
    thread {
        Thread.sleep(100)
        latch.complete(Unit)
        println("Latch lowered")
    }
    latch.await()
}

fun runSomeTestThing(dispatcher: CoroutineDispatcher) {
    GlobalScope.launch(dispatcher) {
        try {
            println("Starting a new task")
            waitForSomethingAsynchronousToHappen()
        } finally {
            println("Cleaning up")
        }
    }
}

fun main() {
    val dispatcher = StandardTestDispatcher()
    TestScope(dispatcher).runTest {
        runSomeTestThing(dispatcher)
    }
    println("Test finished")
    Thread.sleep(300)
    println("Tired of waiting")
}

prints

Starting a new task
Test finished
Latch lowered
Tired of waiting

The cleanup never runs.