Removed xfail from app submission tests and added environment parameter of MockApplication
The parameter is used when setting the submission id and security code, I couldn't use the same submission id for both dev and stage. The staging submission id was generated separately.

https://github.com/mozilla/marketplace-tests/issues/425
Merge pull request #426 from chirarobert/unxfail

Removed xfail from app submission tests and added environment parameter ...