Adding spec to test basic forking functionalities

What does this MR do?

This PR implements task Add a new scenario to test basic forking functionalities.

Steps:

1. Log-in with root
2. Create a public project (we already have a factory for that)
3. Sign-out
4. Sign-up with a new user
5. Fork the project created in 2.
6. Clone the fork's repo and push changes to the fork's repo (we already have a factory for that)
7. Submit a merge request to the origin project (we already have a factory for that)
8. Sign-out
9. Log-in with root
10. Accept the merge request

It also removes /qa/specs/features/login/standard_spec.rb since the verification from that spec is now done on each login step.

Why was this MR needed?

This PR increases automated test coverage.

What are the relevant issue numbers?

gitlab-qa#264 (closed)