test: install the EXAMPLE service through a fixture, not the live config

Two tests failed when the local unshackle.yaml left out the in-repo services folder.

tests/conftest.py

@@ -69,6 +69,17 @@ def no_speed_limit(monkeypatch: pytest.MonkeyPatch) -> None:
     monkeypatch.setattr(requests_downloader, "speed_limit_locked", False)
 
 
+@pytest.fixture
+def example_service(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Make the in-repo EXAMPLE the only installed service, whatever the local unshackle.yaml lists."""
+    from unshackle.core import services
+
+    path = config.directories.namespace_dir / "services" / "EXAMPLE" / "__init__.py"
+    monkeypatch.setattr(services, "SERVICES", [path])
+    monkeypatch.setattr(services, "MODULES", {})
+    monkeypatch.setattr(services, "LOAD_ERRORS", [])
+
+
 @pytest.fixture
 def mux_args(monkeypatch: pytest.MonkeyPatch) -> Callable[..., list[str]]:
     """Run Tracks.mux up to mkvmerge and return its arguments; ``ids`` is what the track ID probe reports."""