Fix crash on repeated MockWebServer shutdown

The problem was the awaitIdle() call was scheduling executor jobs after
the ExecutorService had been shut down. This fixes that and defends
against other races.

okhttp-testing-support/src/main/java/okhttp3/OkHttpClientTestRule.kt

@@ -87,7 +87,7 @@ class OkHttpClientTestRule : TestRule {
 
   private fun ensureAllTaskQueuesIdle() {
     for (queue in TaskRunner.INSTANCE.activeQueues()) {
-      assertThat(queue.awaitIdle(TimeUnit.MILLISECONDS.toNanos(1000L)))
+      assertThat(queue.idleLatch().await(1_000L, TimeUnit.MILLISECONDS))
           .withFailMessage("Queue still active after 1000 ms")
           .isTrue()
     }