Fix broken tests.

* slime-tests.el (inspector): Use `cl-assert`.

* slime.el (slime-with-popup-buffer): Must return current buffer.
(slime-kill-all-buffers): Fix FIXME.
(slime-highlight-differences-in-dirname): Use `split-string' not
`slime-split-string'.

* contrib/slime-repl.el (slime-kill-all-buffers): Move it here where it's
needed.
(slime-switch-to-output-buffer): Use `pop-to-buffer'