Test was failing because family mapping is loaded while connection is
already closed.

```
Traceback (most recent call last):
  File "erp5/Products/ERP5/tests/testInvalidationBug.py", line 156, in testLateInvalidationFromZEO
    activity_tool.tic()
  File "erp5/product/ERP5Type/tests/ProcessingNodeTestCase.py", line 96, in tic
    self._orig_tic(processing_node, force)
  File "erp5/product/CMFActivity/ActivityTool.py", line 1326, in tic
    node_family_id_set = self.getCurrentNodeFamilyIdSet()
  File "erp5/product/CMFActivity/ActivityTool.py", line 921, in getCurrentNodeFamilyIdSet
    return self._getNodeFamilyIdDict().get(getCurrentNode(), ())
  File "ZODB/Connection.py", line 909, in setstate
    self._setstate(obj, oid)
  File "ZODB/Connection.py", line 953, in _setstate
    p, serial = self._storage.load(oid, '')
  File "ZEO/ClientStorage.py", line 825, in load
    result = self.loadBefore(oid, m64)
  File "ZEO/ClientStorage.py", line 839, in loadBefore
    raise ClientDisconnected()
ClientDisconnected
```

Load the object explicitly before so that it's in cache.

/reviewed-on nexedi/erp5!881