Write spec to verify FeatureFlagReferenceFilter does not have N+1

Spun from #327630 (closed)

A spec needs to be added to verify that FeatureFlagReferenceFilter does not have an N+1 problem. See Check for N+1 in ExternalIssueReferenceFilter (!69222 - merged) for an example of what needs to get added.

Implementation Guide

See Check for N+1 in ExternalIssueReferenceFilter (!69222 - merged) and Check for N+1 in IssueReferenceFilter (!157110 - merged) for an example of what needs to get added.