Resolve "Extract EE specific files/lines for project_creator_spec"

What does this MR do?

0. Implement expect_next_instance_of so we could use it to:
1. Resolve "Extract EE specific files/lines for project_creator_spec"

Are there points in the code the reviewer needs to double check?

CE at: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19850

We should maybe rewrite all expect_any_instance_of but it's probably pretty minor and we could just do it along the way. Should we document this?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• EE specific content should be in the top level /ee folder

What are the relevant issue numbers?

Closes #6436 (closed), #6149 (closed)