The getter compares the current entry index against the entry
list's size, but valid indices are in the range [0, size - 1].
An index equal to size is a past-the-end state that never occurs
in practice, so the function always returns true for any valid
index. This means it reports forward navigation is possible even
when the current entry is the last one in the list, where
forward() immediately throws an InvalidStateError.
The symmetric can_go_back() correctly checks against index 0,
and forward() correctly checks against size - 1. This brings
can_go_forward() in line with both.
The invariants for these property getters are supposed to be checked by
the has_entries_and_events_disabled AO, but we don't have all the
plumbing hooked up between Navigables and Navigation yet.
Add a test to make sure that these methods don't assert when calling
them on a fresh page.
