Adapt the tests

Make the failing tests pass by manually setting _is_loaded to True
to mimic that the widget is correctly loaded so that
the add of overlays can be done

Author: bmatthieu3

src/tests/test_aladin.py

@@ -15,6 +15,11 @@
 
 aladin = Aladin()
 
+# Tell python that the JS part is loaded
+# so that adding overlay requests are sent
+# through the events/traitlets system.
+aladin._is_loaded = True
+
 
 # monkeypatched sesame call to avoid remote access during tests
 @pytest.fixture