Resolve "Flaky test in spec/workers/merge_requests/assignees_change_worker_spec.rb"

What does this MR do?

Addresses a flaky spec by ignoring array order in an expectation.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is not a user-facing change.
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Related to #327144 (closed)