Raise exception if ADR_CONFIG is not set for integration tests

When running pytest against the integration tests we need to set ADR_CONFIG as an environment variable manually.
This warns the user that the variable is required.

tests/test_integration.py

@@ -13,6 +13,9 @@ pytestmark = pytest.mark.skipif(
     os.environ.get("TRAVIS_EVENT_TYPE") != "cron", reason="Not run by a cron task"
 )
 
+if not os.environ.get("ADR_CONFIG_PATH"):
+    raise Exception("Set ADR_CONFIG_PATH to tests/config.toml")
+
 
 @pytest.fixture
 def adr_config(tmp_path):