add test for #5395

[Kilowhisky]

Adds failing test for #5395

[runspired]

As ember has deprecated array observers in 4.0, and as there are many patterns for reacting to record-array changes efficiently without array-observers, and as the relationship layer has been refactored to generally be far more efficient causing over notifications to go down (the source of some of the trouble leading to the issue for which this PR was opened), and as the craziness of unloadRecord is mostly being eliminated due to internal conversion to identifiers (such that unloaded records can always be fully released), I'm going to close this.

Long winded I know but that's because fundamentally I care about why this PR was opened in the first place: performant array operations matter a lot. There are a lot of good patterns out there though that don't require the array-observer, and we're dedicated to improving base performance where possible. If @Kilowhisky or anyone else watching this issue still faces performance problems using octane paradigms and ember-data 3.28+ please reach out. At a minimum we should create a performance benchmark to add to the performance tests we run in CI so that we can monitor and focus on improving the base performance of the scenario, and if it's evident more is needed to achieve the necessary performance characteristics for large data sets then we'll look into what we can do to make subscribing to granular updates for a type first-class.