Support cleanly multiple clients in the tests

[LewisGaul]

• [ ] test_tasks.py doesn't use the docker_client fixture
• [ ] the tests in the buildx directory don't use the docker_client fixture.
• [ ] test_compose.py doesn't use the docker_client fixture (technically it can't but we should be able to transfer the --docker-exe arg)
• [x] Enable strict xfail globally.

Originally posted by @gabrieldemarmiesse in https://github.com/gabrieldemarmiesse/python-on-whales/issues/353#issuecomment-1903979656