Add failing test for #5167

[bantic]

Demonstrates the behavior described in #5167.

Removing these lines from loadCar (and the assertions of the value of person.get('cars.length'))
makes the tests pass:

  run(() => {
    env.store.peekAll('car').forEach(car => car.set('person', person));
  });

[bantic]

Added comments in the test to point out which assertions fail.

[runspired]

This is covered by the tests added in #5378