Transition Acceptance tests over to a real GitLab instance

[Shocktrooper]

While coding a new part of this project I noticed that the tests are coded against a http server vs a real GitLab instance. In order to keep the connectors up to date and weed out any breaking changes in the GitLab API as GitLab releases new versions I propose we change the tests to work against a real GitLab server. The downstream GitLab terraform provider already does this for its tests and I can see the same benefits that that project gets being realized for this one. The only small blocker I can see is that we would need an EE license for some of the tests that encapsulate EE only capabilities but that shouldn't be that hard to get.