Tests, WebHost: enable Flask DEBUG mode for tests

This provides the actual error if a test raised an exception on the server.

test/webhost/__init__.py

@@ -22,6 +22,7 @@ def setUpClass(cls) -> None:
         }
         raw_app.config.update({
             "TESTING": True,
+            "DEBUG": True,
         })
         try:
             cls.app = get_app()