Enhance NewGitlabDomainsSourceStub for acceptance tests

Acceptance tests for gitlab source are easy to build but it's hard to test some things like changing URLs for zip artifacts that depend on a mock server.

An idea is to generate a httptest.NewServer() that will return a valid URL with an available port. This URL would need to be injected somehow in the the stubbed response.

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The following discussion from !351 (merged) should be addressed:

• @ayufan started a discussion: (+1 comment)

  Do we need that? Can we just do httptest.NewServer()?