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

What does this MR do and why?

This change adds a new test case to verify that when a GitLab-Slack integration is duplicated at the instance level (meaning it applies to the entire GitLab installation rather than just a specific project), the system automatically sets a default alias name of "_gitlab-instance" for the Slack integration.

Verification

✅ A green pipeline confirms the new tests pass and no regressions were introduced. The test ensures that when duplicating an instance-level Slack integration, the alias correctly defaults to '_gitlab-instance'.

MR acceptance checklist

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Related to #568958 (closed)