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/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