Fix approval spec for project with only one member
This replicates the solution from https://gitlab.com/gitlab-org/gitlab-ee/commit/ac277b810b5aed6dbaf4bd3efab5beca732d0ca1 to make sure that the project really has only one member.
The second context
block doesn't make sense, because if there's only one user in the project, then it has to be the same user as the merge request author.
/cc @mkozono