Do not use hardcoded group path in tests

What does this MR do and why?

Fix failures from gitlab-org/quality/engineering-productivity/master-broken-incidents#606 (closed).

In https://gitlab.com/gitlab-org/gitlab/-/blob/c0f1be66fe74d9fef70f391ccd70d8c3f7c4c74b/ee/spec/services/epics/related_epic_links/list_service_spec.rb#L87 we use hard-coded group path, which leads to the following difference (group1 vs group3):

(byebug) epic_response(public_epic, relation_path1).slice(:id, :relation_path)
{:id=>46, :relation_path=>"/groups/group1/-/epics/1/related_epic_links/34"}
(byebug) subject[0].slice(:id, :relation_path)
{:id=>46, :relation_path=>"/groups/group3/-/epics/1/related_epic_links/34"}

This happens when there is group already created in another test (and persisted with let_it_be), so it gets the group1 path, and the one from the failing spec gets a different one, e.g. group3.

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How to set up and validate locally

On current master branch run

bundle exec rspec ./ee/spec/models/audit_events/external_audit_event_destination_spec.rb ./ee/spec/services/epics/related_epic_links/list_service_spec.rb

There will be failures, as described in the incident issue.

When the same tests are executed on this branch, failures are gone.

MR acceptance checklist

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