[devtools] Avoid db_cache collisions and mis-loading This is done by stopping to share the db_cache between TestDataBaseHandler instances It may seem overkill, but is the only way I could get it to work properly. I think the whole db caching code needs some rework to better work with different db handlers. Closes #4601328

[system]
db-driver   = sqlite
adapter     = native
db-name     = tmpdb-multi
db-encoding = UTF-8