[TEST] wait for threads to finish writing before checking

we need to make sure we wait for all threads to finish executing, since there might still be a thread around even post await (i.e. just starting) performing updates

src/test/java/org/elasticsearch/common/collect/UpdateInPlaceMapTests.java

@@ -125,6 +125,10 @@ public void run() {
 
         numberOfMutations.await();
 
+        for (Thread thread : threads) {
+            thread.join();
+        }
+
         // verify the 2 maps are the same
         assertThat(Iterables.toArray(map.values(), String.class), arrayContainingInAnyOrder(Iterables.toArray(verifier.values(), String.class)));
     }