Clarify existing test_extend_and_override test
Clarify existing test_extend_and_override test

I found it confusing that the used `block` was not used/checked via the
mark, and therefore added explicit asserts for it.

EliotBerriot/django-app-namespace-template-loader,Fantomas42/django-app-namespace-template-loader,bittner/django-app-namespace-template-loader,Fantomas42/django-app-namespace-template-loader,EliotBerriot/django-app-namespace-template-loader,bittner/django-app-namespace-template-loader