Refactor the tests to use --run-docker and --run-podman

Simplified test setup using a parameterised fixture. The VFS test
is now just a single test that handles its own clean up.

unit_test/conftest.py

@@ -8,20 +8,7 @@ MOCK_PACKAGE_DIR = Path("some_package_dir")
 
 def pytest_addoption(parser):
     parser.addoption("--run-docker", action="store_true", default=False, help="run docker tests")
-
-
-def pytest_configure(config):
-    config.addinivalue_line("markers", "docker: mark test requiring docker to run")
-
-
-def pytest_collection_modifyitems(config, items):
-    if config.getoption("--run-docker"):
-        # --run-docker given in cli: do not skip container tests
-        return
-    skip_docker = pytest.mark.skip(reason="need --run-docker option to run")
-    for item in items:
-        if "docker" in item.keywords:
-            item.add_marker(skip_docker)
+    parser.addoption("--run-podman", action="store_true", default=False, help="run podman tests")
 
 
 @pytest.fixture