Fix bug involving stale window-end.

Call window-end with update=t.  That works better than calling
redisplay when we scroll because window-end may be wrong even
without scrolling.

* slime.el (slime--adjust-window-point): Call window-end with
update argument.
(slime--adjust-window-start): Remove explicit call to redisplay.
* slime-tests.el (display-region): Actually check point ends up as
expected.  The old code was buggy and only checked for non-nil.
Stricter assertion

LAST should be visible as long as
REGION-HEIGHT <= WINDOW-HEIGHT not <.