Rewrite frontend checkout sources specs
Rewrite frontend checkout sources specs

They rely on the database to be filled to check things
happened correctly. This is a bad practice and we can easily check
the same thing by checking the output of the page.

This should fix this flaky spec: https://circleci.com/gh/nebulab/solidus/1726#tests/containers/2

pervino/solidus,pervino/solidus,pervino/solidus,pervino/solidus