system: Add clarification to System.addressOf()

SpiderMonkey can move objects around in memory during garbage collection,
so System.addressOf() cannot be relied on to identify that an object is
the same as another object.

At the same time, SpiderMonkey 38 also gained the capability to
deduplicate identical objects in memory, so System.addressOf() can also
not be relied on to identify that two identical objects are different
instances.

Adjust our test suite accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=776966