Added pytest fixture with app factory

I forgot about the app fixture we're using in all of the plugins :(

Some explanation (if necessary):

• Factory creates an app with registered blueprint, like here https://stackoverflow.com/a/51663667
• Then it's used as a plugin, e.g. https://gist.github.com/peterhurford/09f7dcda0ab04b95c026c60fa49c2a68#gistcomment-2371025