Signup: update signup tests as some flows update the step order

[taggon]

This PR moves back the user step related to testing steps since the user step becomes the last step in some flows because of https://github.com/Automattic/wp-calypso/pull/30696.

[southp]

@taggon Since we now have a place to share the same step testing code , it would be a great step forward if we could move the user step testing code there and reuse it among test suites :)