Detect leaks of EngineSession / GeckoSession

[pocmo]

Follow-up from #3888. We now have the ability to detect leaks during instrumented tests. By default leak canary will detect context, activity, fragment leaks. However a big category of leaks that we would want to avoid is leaks of EngineSession / GeckoSession instances. Let's see if we can add them to the leak detection.

┆Issue is synchronized with this Jira Task

[csadilek]

Moved to bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1794936

Change performed by the Move to Bugzilla add-on.