Add spec to test preloader in audit events report service

What does this MR do and why?

resolves: #390533 (closed)

It was mentioned in a separate MR that the preloader was not being tested.

This commit adds a spec that does that.

If the preloader lines is removed, all of the existing specs still pass however the new spec fails with the following message:

Failure/Error: expect { described_class.new(params).csv_data.to_a }.not_to exceed_query_limit(control)

  Expected a maximum of 1 queries, got 2:

  Query Diff:
  -----------
  SELECT "users"."id", "users"."name", "users"."username" FROM "users"...
  -- (expected: 0, got: 1)
     WHERE "users"."id" IN (16, 18) ORDER BY "users"."id" ASC LIMIT 1000

Screenshots or screen recordings

N/A

How to set up and validate locally

1. remove preloader line here
2. run specs, the new spec should fail
3. re-add the line. all specs should pass

MR acceptance checklist

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