Improve objects lifetimes tests once we have a write API

[lawrence-forooghian]

In #26 we introduced some objects lifetimes tests. There are some improvements that I'd like to make once we have a write API, thus making it easy to populate the ObjectsPool and emit object updates:

• check that there is stable object identity for non-root objects
• in the tests of "objects functionality works when user just has a strong reference to public $x$", check that we also receive object update events as part of this "working"

┆Issue is synchronized with this Jira Task by Unito