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".

tests/data/extensions/devices/test_device.py

@@ -17,7 +17,7 @@
 from wa import Plugin
 
 
-class TestDevice(Plugin):
+class MockDevice(Plugin):
 
     name = 'test-device'
     kind = 'device'