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/dbus_middleware1/bus_provider.py

@@ -0,0 +1,36 @@
+# -*- coding: utf-8 -*-
+# SPDX-FileCopyrightText: 2025 KUNBUS GmbH
+# SPDX-License-Identifier: GPL-2.0-or-later
+from time import sleep
+
+from tests.dbus_middleware1.fake_devices import PiControlDeviceMockup
+
+
+class TestBusProvider(PiControlDeviceMockup):
+
+    def setUp(self):
+        super().setUp()
+
+        # Do not import things on top of the module. Some classes or functions need to be mocked up first.
+        from revpi_middleware.dbus_middleware1 import BusProvider
+
+        # Prepare the bus provider and start it
+        self.bp = BusProvider(
+            self.picontrol.name,
+            use_system_bus=False,
+        )
+        self.bp.start()
+
+        # Wait 5 seconds until the bus provider has started the main loop
+        counter = 50
+        while not self.bp.running and counter > 0:
+            counter -= 1
+            sleep(0.1)
+
+    def tearDown(self):
+        self.bp.stop()
+        self.bp.join(10.0)
+        if self.bp.is_alive():
+            raise RuntimeError("Bus provider thread is still running")
+
+        super().tearDown()