[Extensions] Reenable test now that load on demand works
[Extensions] Make instance destruction test stricter

In the test, we wanted to ensure that instances (at the time "Contexts")
were destroyed correctly. But we were creating more instances than we
were using, so the checks were less strict.

Now that we only create an instance if the extension is actually used,
we can make the ASSERTs stricter.
Merge pull request #1208 from cmarcelo/updating-tests-after-load-on-demand-landed

[Extensions] Updating tests after load on demand landed