prevent race condition in purge_results test

There were some cases where the Hub could be too slow noticing that the result was finished, causing an error saying that pending messages cannot be purged.

IPython/parallel/tests/test_client.py

@@ -249,7 +249,14 b' class TestClient(ClusterTestCase):'
         # ensure there are some tasks
         for i in range(5):
             self.client[:].apply_sync(lambda : 1)
+        # Wait for the Hub to realise the result is done:
+        # This prevents a race condition, where we
+        # might purge a result the Hub still thinks is pending.
+        time.sleep(0.1)
+        rc2 = clientmod.Client(profile='iptest')
         hist = self.client.hub_history()
+        ahr = rc2.get_result([hist[-1]])
+        ahr.wait(10)
         self.client.purge_results(hist[-1])
         newhist = self.client.hub_history()
         self.assertEquals(len(newhist)+1,len(hist))