Fixes a potential race condition with logging

This fixes a potential race condition where a test runs a thread in
the executor that does some logging. Without waiting for threads to
finish, another test can be run while that background thread is still
executing. This can cause flaky issues with tests that only think
that their test code is executing.

Author: Kainsin

okhttp/src/test/java/io/grpc/okhttp/OkHttpClientTransportTest.java

@@ -192,7 +192,12 @@ public class OkHttpClientTransportTest {
 
   @After
   public void tearDown() {
-    executor.shutdownNow();
+    try {
+      executor.shutdownNow();
+      executor.awaitTermination(10, TimeUnit.SECONDS);
+    } catch (InterruptedException e) {
+        // Ignore in a test and continue on as normal.
+    }
   }
 
   private void initTransport() throws Exception {