test(dbus): Add unit test framework for dbus_middleware1 module

Introduces test infrastructure and mockup classes for
`dbus_middleware1`. Includes `BusProvider` test setup, fake device
implementations, and initialization logic to support session bus
testing. Enhances testability and isolation of the D-Bus middleware
components.

tests/__init__.py

@@ -0,0 +1,3 @@
+# -*- coding: utf-8 -*-
+# SPDX-FileCopyrightText: 2025 KUNBUS GmbH
+# SPDX-License-Identifier: GPL-2.0-or-later