This way on plain ZODB4 the following non-wcfs tests will continue to
pass

   test.py/fs-!wcfs
   test.py/zeo-!wcfs
   test.py/neo-!wcfs

instead of failing as e.g. in here:

   https://nexedijs.erp5.net/#/test_result_module/20211116-123A66706

On plain ZODB4 WCFS-related functionality - which uses zconn_at - will
continue to raise corresponding assertion in WCFS-related tests, as e.g. in

https://nexedijs.erp5.net/#/test_result_module/20211116-123A66706/6