It is better if the various tests share as little state as possible, so make sure they all have their own Electrum ABC daemon with a fresh wallet and datadir.
Restrict the scope fulcrum_service fixture to function, create and delete the datadir for every test function.
This removes a global variable.
Also note that the bitcoind_rpc_connection already assigns the _bitcoind global variable the first time it is called, so there is no need to redo it in fulcrum_service.
It would be good to also restart/reset the bitcoind instance and the fulcrum instance between tests, but I was not able to find any pytest-docker documentation about how to do that. The indivudual tests can mine blocks and send funds from the bitcoind wallet, so there is still some shared state between tests.