Allow partially visible lines when scrolling.

[github issue #186]

* slime.el (slime--adjust-window-start)
(slime--adjust-window-point): Relax the assertion to allow
partially vsible lines.
Factor out the backquote test.

* slime-tests.el (slime-test--compile-defun): New helper.
([test] compile-defun, compile-defun-with-reader-conditionals):
Use it.
([test] compile-defun-with-backquote): New.