checkPackUndoLog(): Use snooze() twice to separate the pack time from the

transactions on both sides of it.  The coarse granularity of time.time()
on Windows causes this test to fail sporadically otherwise.

src/ZODB/tests/PackableStorage.py

@@ -392,9 +392,9 @@ class PackableStorage(PackableStorageBase):
         # Commit two different revisions
         revid1 = self._dostoreNP(oid, data=pickle.dumps(obj))
         obj.value = 2
-        now = packtime = time.time()
-        while packtime <= now:
-            packtime = time.time()
+        snooze()
+        packtime = time.time()
+        snooze()
         revid2 = self._dostoreNP(oid, revid=revid1, data=pickle.dumps(obj))
         # Now pack the first transaction
         self.assertEqual(3,len(self._storage.undoLog()))