Relax assertion in check_tid_ordering_w_commit test

It is pointless for lastTransaction() to block until it is allowed to
return the TID of a transaction that has just been committed, because
it may still not be the real last TID (e.g. for some storage
implementations, invalidations are received from a shared server
via the network). While invalidations are still being processed,
it's fine to return immediately with the previous last TID.

This was clarified in commit 4a6b0283
("mvccadapter: check if the last TID changed without invalidation").

See pull request #316

src/ZODB/tests/BasicStorage.py

@@ -347,17 +347,13 @@ class BasicStorage(object):
                 attempts_cond.notifyAll()
 
 
-        @run_in_thread
-        def lastTransaction():
-            update_attempts()
-            results['lastTransaction'] = self._storage.lastTransaction()
-
         @run_in_thread
         def load():
             update_attempts()
             results['load'] = utils.load_current(self._storage, ZERO)[1]
+            results['lastTransaction'] = self._storage.lastTransaction()
 
-        expected_attempts = 2
+        expected_attempts = 1
 
         if hasattr(self._storage, 'getTid'):
             expected_attempts += 1