Add end-to-end tests for the daemon

Each test runs xidle-inhibit on a private dbus-daemon with fake lockers on PATH,
covering lock blocking, cookie handling, restoring after restarts and cleanup.

pyproject.toml

@@ -7,5 +7,8 @@ select = ["E", "F", "W", "I", "B", "UP"]
 
 [tool.mypy]
 # Scripts both map to __main__, so check xidle-dim separately: mypy xidle-dim
-files = ["xidle-inhibit"]
+files = ["xidle-inhibit", "tests"]
 strict = true
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]