Fixed insideous test failure

A test was counting on a cache being written to, but it wasn't because
the storage got an invalidation later than expected (on suitably slow
machines, like travis and an AWS dev machine of mine).

src/ZEO/tests/ConnectionTests.py

@@ -609,6 +609,10 @@ class InvqTests(CommonSetupTearDown):
         revid2 = self._dostore(oid2)
         revid2 = self._dostore(oid2, revid2)
 
+        forker.wait_until(
+            lambda :
+            perstorage.lastTransaction() == self._storage.lastTransaction())
+
         perstorage.load(oid, '')
         perstorage.close()