Follow-up from "Fix SlackIntegration duplicate bug when inheriting from parent"

The following discussion from !203590 (merged) should be addressed:

• @SamWord started a discussion:

  Sorry I should have been more specific sooner, the spec context that I'd like to see is one where .dup/.after_build_from_integration results in setting the alias to '_gitlab-instance'.

  It's not represented in the specs because new_integration has a project parent. So even though the spec is doing what it should by copying new_integration's SlackIntegration, there's no spec capturing what happens when duplicating a slack app instance integration (the right side of this || isn't covered)

  In reality, it isn't worth blocking this MR over because we wouldn't be calling .dup on an instance slack app integration, and it's pretty obvious from code review that it's covered for now, so a spec is just something small for regressions in the future. This context can be added in a tiny follow-up MR 👍