tests: fix pytest warnings

Fix warnings reported when running unit tests via pytest.

- Rename TestDevice to MockDevice, so that it is not interpreted as a
  test case.
- Fix collections abstract base class imports.
- Add an ini file to ignore the same from "past".

pytest.ini

@@ -0,0 +1,3 @@
+[pytest]
+filterwarnings=
+    ignore::DeprecationWarning:past[.*]