Improve test env

* Replace big gitlabhq repo with small one (https://gitlab.com/gitlab-org/gitlab-test)
* dont stub gitlab-shell calls

Advantages:

* test gitlab-shell install during tests
* test integration with gitlab-shell features (fork, change default branch etc)
* don't store archive with test repo inside gitlab
* less stubs - easier to write/read tests

During this refactoring __next issues are be fixed__:

* satellite error if gitlab_shell path is relative
* project rename repo -> 500 error
* fixed wrong diff for MR `download plain diff`
* fixed bug when gfm ignores `@ref` variable during link parsing

Fixes #1451

See merge request !1005