Fix the invalidation-during-load test to execute the branch of interest.

src/ZODB/tests/testmvcc.py

@@ -256,7 +256,10 @@ to test the Connection code.
 ...            self.db.invalidate(tid, {oid:1})
 ...            self.count += 1
 
-Now we'll repeat all the setup that was done earlier.
+We can execute this test with a single connection, because we're
+synthesizing the invalidation that is normally generated by the second
+connection.  We need to create two revisions so that there is a
+non-current revision to load.
 
 >>> ts = TestStorage()
 >>> db = DB(ts)
@@ -266,6 +269,8 @@ Now we'll repeat all the setup that was done earlier.
 >>> r1["a"] = MinPO(0)
 >>> r1["b"] = MinPO(0)
 >>> cn1.getTransaction().commit()
+>>> r1["b"].value = 1
+>>> cn1.getTransaction().commit()
 >>> cn1.cacheMinimize()
 
 >>> oid = r1["b"]._p_oid
@@ -280,9 +285,6 @@ False
 >>> r1["b"]._p_state
 -1
 >>> r1["b"]._p_activate()
-Traceback (most recent call last):
-  ...
-ReadConflictError: database read conflict error (oid 0000000000000002, class ZODB.tests.MinPO.MinPO)
 >>> oid in cn1._invalidated
 True
 >>> ts.count
@@ -290,9 +292,6 @@ True
 
 _p_independent() still has the desired effect.
 
-We still get a non-current version if the invalidation occurs while we
-are loading the current revision.  Can that happen without ZEO?
-
 Error cases:
 - storage doesn't have an earlier revision
 - MVCC returns current revision