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."""

tests/remote/unit/test_handler_proxy_policy.py

@@ -48,6 +48,7 @@ def test_region_mismatch_raises(monkeypatch):
     assert exc_info.value.details == {"service": "EXAMPLE"}
 
 
+@pytest.mark.usefixtures("example_service")
 def test_geoblocked_client_region_raises(monkeypatch):
     patch_server_region(monkeypatch, "CA")
     service_class = handlers.Services.load("EXAMPLE")

tests/remote/unit/test_routes.py

@@ -232,6 +232,7 @@ async def test_heartbeat_late_error_counts_as_rejected(aiohttp_client, monkeypat
     assert stats.errors == {"-": {"SERVICE_ERROR": 1}}
 
 
+@pytest.mark.usefixtures("example_service")
 @pytest.mark.parametrize(("service", "counted_as"), [("EXAMPLE", "EXAMPLE"), ("NOT_INSTALLED", "-")])
 async def test_error_is_counted_by_service_and_logged_with_its_ref(
     aiohttp_client, monkeypatch: pytest.MonkeyPatch, caplog: pytest.LogCaptureFixture, service: str, counted_as: str