tests: move testing stuff into its own directory

testing/conftest.py

@@ -0,0 +1,11 @@
+from pytest import Config
+
+
+def pytest_configure(config: Config):
+    """
+    Registers a new mark 'final' that can be applied to tests.
+
+    I did not want to add these in pyproject.toml.
+    """
+
+    config.addinivalue_line("markers", "final: the final test")