checkTwoArgBegin(): Change the explicit tid that gets stored so that

the Standby storage version of the test has a hope of succeeding.
That storage ignores non-sensically out-of-sequence tids so as written
this test wouldn't have passed until after 3220-07-29 10:18:09.882353
<methuselah wink>.

We'll use a tid that we know is earlier than now.

src/ZODB/tests/BasicStorage.py

@@ -177,7 +177,7 @@ class BasicStorage:
     def checkTwoArgBegin(self):
         # XXX how standard is three-argument tpc_begin()?
         t = Transaction()
-        tid = chr(42) * 8
+        tid = '\0\0\0\0\0psu'
         self._storage.tpc_begin(t, tid)
         oid = self._storage.new_oid()
         data = zodb_pickle(MinPO(8))