Refactor and test reset_merge_request_approvals

What does this MR do and why?

This MR

• Changes the definition of conditions in app/policies/base_policy.rb for consistency with others. It doesn't change its behavior.
• Adds tests for reset_merge_request_approvals ability.
• Moves tests "PUT :id/merge_requests/:merge_request_iid/reset_approvals" from spec/requests/api/merge_requests_spec.rb to spec/requests/api/merge_request_approvals_spec.rb and improves them.

Related to !95525 (merged), !104496 (merged)

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