Added failing test: "Fetching a belongsTo that is set to null removes the record from a relationship - sync - and do it twice"

[eodb]

This is a PR to facilitate the fixing of https://github.com/emberjs/data/issues/8990. An extra test "Fetching a belongsTo that is set to null removes the record from a relationship - sync - and do it twice", has been added.

Description

It is actually the same test as the existing one that precedes it: "Fetching a belongsTo that is set to null removes the record from a relationship - sync" but only run twice immediately after each other. If the user.accounts are not accessed in between (see the assert.strictEqual - put in comment), the test fails, otherwise the test succeeds.