When creating a "new_instance", copy loadBefore, as well s pack.

This is needed because pack temporarily deletes data from the data
structure that loadBefore reads.

Note that in the new scheme of things, we avoid using load, but
instead use loadBefore.  This we bypass the _data_snapshot
shenanigans, which makes me think we aren't testing what we should be,
but I have no confidence that the machinery in MVCCMappingStorage
actually reflects anything meaningful.

src/ZODB/tests/MVCCMappingStorage.py

@@ -46,6 +46,7 @@ class MVCCMappingStorage(MappingStorage):
         inst._commit_lock = self._commit_lock
         inst.new_oid = self.new_oid
         inst.pack = self.pack
+        inst.loadBefore = self.loadBefore
         inst._main_lock_acquire = self._lock_acquire
         inst._main_lock_release = self._lock_release
         return inst