Refactor and re-enable test for failed pre import

Context

The following discussion from !955 (merged) should be addressed:

• @jaime started a discussion: (+2 comments)

  @jdrpereira we need to rewrite TestGitlabAPI_RepositoryImport_Put_PreImportFailed since it was expecting the pre import to fail but now it doesn't. I'll try to find a solution a bit later today.

Problem

This API test was relying on a scenario that is no longer possible to simulate after the fix introduced in !955 (merged). There isn't clear/easy option to patch this test, as we're very limited in what we can do at the API level. We disabled the test in !955 (merged) but it provides value so we should find a way to re-enable it.