Fix flaky test `lazy_user_notes_count_aggregate_spec.rb`

What does this MR do and why?

Modifies the lazy_user_notes_count_aggregate_spec.rb spec to de-duplicate the expected array of ids.

This is because, as currently written, if FactoryBot assigns an id of 20 (or 10 or 30) to the vulnerability, the spec will fail, making this a flaky spec.

You can see my comment on the issue for a more detailed breakdown of the flakiness

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Hard-code an id of 20 for the vulnerability in the spec

   diff --git a/ee/spec/lib/gitlab/graphql/aggregations/vulnerabilities/lazy_user_notes_count_aggregate_spec.rb b/ee/spec/lib/gitlab/graphql/aggregations/vulnerabilities/lazy_user_notes_count_aggregate_spec.rb
   index 43d528d1feb4..19aed5ee4120 100644
   --- a/ee/spec/lib/gitlab/graphql/aggregations/vulnerabilities/lazy_user_notes_count_aggregate_spec.rb
   +++ b/ee/spec/lib/gitlab/graphql/aggregations/vulnerabilities/lazy_user_notes_count_aggregate_spec.rb
   @@ -7,7 +7,7 @@
        {}
      end
    
   -  let(:vulnerability) { create(:vulnerability) }
   +  let(:vulnerability) { create(:vulnerability, id: 20) }
    
      describe '#initialize' do
        it 'adds the vulnerability to the lazy state' do
       ```

2. running the spec without this patch will result in failure:

   Failures:
   
     1) Gitlab::Graphql::Aggregations::Vulnerabilities::LazyUserNotesCountAggregate#initialize when there are existing pending_vulnerability_ids uses lazy_user_notes_count_aggregate to collect aggregates
        Failure/Error: expect(result.lazy_state[:pending_vulnerability_ids]).to match_array expected_ids
        
          expected collection contained:  [10, 20, 20, 30]
          actual collection contained:    [10, 20, 30]
          the missing elements were:      [20]
        # ./ee/spec/lib/gitlab/graphql/aggregations/vulnerabilities/lazy_user_notes_count_aggregate_spec.rb:35:in `block (4 levels) in <top (required)>'
        # ./spec/spec_helper.rb:435:in `block (3 levels) in <top (required)>'
        # ./spec/support/sidekiq_middleware.rb:9:in `with_sidekiq_server_middleware'
        # ./spec/spec_helper.rb:426:in `block (2 levels) in <top (required)>'

3. running the spec with this patch should all pass

Related to Failure in ee/spec/lib/gitlab/graphql/aggregati... (#441189 - closed) • Michael Becker • 16.10