Tidy up `ctr_client` parameterisation/marking mechanism

[LewisGaul]

• Improve ctr_client parameterisation:
  • Automatically apply docker/podman marks (in pytest_collection_modifyitems() hook)
  • Remove indirection when parameterising ctr_client (no need to import anything from test_utils)
  • Simplify code that gets and checks container client (no need for TestSessionClient class)
  • Support using docker_client and podman_client fixtures directly for testcases that do not need parameterising

First PR factored out of https://github.com/gabrieldemarmiesse/python-on-whales/pull/525