Speed up Project security access specs

Prior, every single test was creating four `ProjectMember` objects, each
of which created one `User` record, even though each test only used
_one_ of those Users, if any.

Now each test only creates the single user record it needs, if it needs
one. This shaves minutes off of each spec file changed here.

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/24899

See merge request !7779