Repair flakiness of bulk_user_assignment_spec's array matcher

What does this MR do and why?

Resolves: https://gitlab.com/gitlab-org/gitlab/-/issues/441511

As described in the issue, ee/spec/lib/duo_pro/bulk_user_assignment_spec.rb#L99 was flaky and was quarantined due to the results not always being returned in the same order. This MR fixes this flakiness by changing the matcher from eq to match_array to directly match the elements in the array regardless of the order.

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.

Screenshots or screen recordings

No frontend changes

How to set up and validate locally

Covered by automated tests

Related to #441511