checkPackAfterUndoDeletion(): the new fix for "redundant" packs caused

trouble here on Windows, due to the poor granulatiry of Windows time.time()
feeding into pack-time selection in this test.  Moved the snooze() from
before pack time selection to after it, which appears to be a sufficient
fix; this can't hurt Linux, because the snooze() calls aren't needed at
all there (but don't hurt either).

src/ZODB/tests/TransactionalUndoStorage.py

@@ -519,8 +519,8 @@ class TransactionalUndoStorage:
 
         pack_times = []
         def set_pack_time():
-            snooze()
             pack_times.append(time.time())
+            snooze()
 
         root["key0"] = MinPO(0)
         root["key1"] = MinPO(1)