Update `GitlabCeRelease` spec to use only local repositories

Depends on !39 (merged)

This drastically speeds up the test because we no longer perform a remote clone of the test repositories.

release-tools rs-release-fixture-repo % be rspec spec/lib/release/gilab_ce_release_spec.rb
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}

Randomized with seed 38889

8< ---------- tons of output ---------- 8<

Finished in 4.97 seconds (files took 0.38784 seconds to load)
8 examples, 0 failures