transfer_service_spec: cleanup, merge common code, check against nil

- replace creation of group2 with the use of group without add_owner(user)
- fold TransferService calls into new test function transfer_project
  - remove currently not used (and not working) gitlab_shell stub
    (will submit testcase simulating failure in gitlab_shell separately)
- add checks against not be_nil (result.should be_false passes even if nil)
Merge branch 'transfer-service-spec-cleanups' into 'master'

Transfer service spec cleanups

Simplify the current tests for Projects::TransferService

See merge request !208