devices: add pytest conftest infra for devices

This is similar to the other `inputs`, `sources` etc conftest.py
modules and allows us to write unit test for the devices code.

devices/test/conftest.py

@@ -0,0 +1,33 @@
+import os
+import pathlib
+from types import ModuleType
+
+import pytest
+
+from osbuild import devices, testutil
+from osbuild.testutil.imports import import_module_from_path
+
+
+@pytest.fixture(name="devices_module")
+def devices_module_fixture(request: pytest.FixtureRequest) -> ModuleType:
+    """devices_module is a fixture that imports a stage module by its name
+    defined in DEVICES_NAME in the test module.
+    """
+    if not hasattr(request.module, "DEVICES_NAME"):
+        raise ValueError("devices_module fixture must be used in a test module that defines DEVICES_NAME")
+
+    devices_name = request.module.DEVICES_NAME
+    caller_dir = pathlib.Path(request.node.fspath).parent
+    module_path = caller_dir.parent / devices_name
+    return import_module_from_path("devices", os.fspath(module_path))
+
+
+@pytest.fixture
+def devices_service(devices_module) -> ModuleType:
+    """devices_service is a fixture that imports a devices module by its name
+    defined in DEVICES_NAME in the test module and returns a Deviceservice
+    """
+    service_cls = testutil.find_one_subclass_in_module(devices_module, devices.DeviceService)
+    fd = testutil.make_fake_service_fd()
+    srv_obj = service_cls.from_args(["--service-fd", str(fd)])
+    return srv_obj