Update 'synchronizers' tests wrt to transaction 1.6.1

Changes are backported from commits 227953b9
and 03a326be.

src/ZODB/tests/synchronizers.txt

@@ -25,11 +25,27 @@ Make a change locally:
     >>> rt = cn.root()
     >>> rt['a'] = 1
 
-Sync should not have been called yet.
+Sync isn't called when a connection is opened, even though that
+implicitly starts a new transaction:
 
-    >>> st.sync_called  # False before 3.4
+    >>> st.sync_called
     False
 
+Sync is called when we explicitly start a new transaction:
+
+    >>> _ = transaction.begin()
+
+    >>> st.sync_called
+    True
+    >>> st.sync_called = False
+
+BTW, calling ``sync()`` on a connection starts a new transaction, which
+caused ``sync()`` to be called on the storage:
+
+    >>> cn.sync()
+    >>> st.sync_called
+    True
+    >>> st.sync_called = False
 
 ``sync()`` is called by the Connection's ``afterCompletion()`` hook after the
 commit completes.
@@ -62,42 +78,14 @@ traceback then ;-)
 
     >>> cn.close()
 
-One more, very obscure.  It was the case that if the first action a new
-threaded transaction manager saw was a ``begin()`` call, then synchronizers
-registered after that in the same transaction weren't communicated to the
-`Transaction` object, and so the synchronizers' ``afterCompletion()`` hooks
-weren't called when the transaction commited.  None of the test suites
-(ZODB's, Zope 2.8's, or Zope3's) caught that, but apparently Zope 3 takes this
-path at some point when serving pages.
+As a special case, if a synchronizer registers while a transaction is
+in flight, then newTransaction and thus the storage sync method is
+called:
 
-    >>> tm = transaction.ThreadTransactionManager()
-    >>> st.sync_called = False
-    >>> dummy = tm.begin()  # we're doing this _before_ opening a connection
-    >>> cn = db.open(transaction_manager=tm)
-    >>> rt = cn.root()      # make a change
-    >>> rt['c'] = 3
-    >>> st.sync_called
-    False
-
-Now ensure that ``cn.afterCompletion() -> st.sync()`` gets called by commit
-despite that the `Connection` registered after the transaction began:
-
-    >>> tm.commit()
-    >>> st.sync_called
-    True
-
-And try the same thing with a non-threaded transaction manager:
-
-    >>> cn.close()
     >>> tm = transaction.TransactionManager()
     >>> st.sync_called = False
-    >>> dummy = tm.begin()  # we're doing this _before_ opening a connection
+    >>> _ = tm.begin()  # we're doing this _before_ opening a connection
     >>> cn = db.open(transaction_manager=tm)
-    >>> rt = cn.root()      # make a change
-    >>> rt['d'] = 4
-    >>> st.sync_called
-    False
-    >>> tm.commit()
     >>> st.sync_called
     True