Don't use deprecated yield fixtures
Don't use deprecated yield fixtures

Advice given to us during the test review [here](https://github.com/raiden-network/raiden/commit/d952a3542004c29c294110b2e9e8a9e85e2196dd#diff-3140c5bd953d5cc1ad34aeacae557b41R15)

hackaugusto/raiden,hackaugusto/raiden