Make slime-repl tests less brittle.

The new tests don't assume that the REPL buffer has actual marker
objects indicating the relative positions of output, result and
prompt. This work is in preparation of handling issue #9.

* slime-repl.el (slime-repl-emit): Propertize the output text with
a `slime-repl-output' boolean property.
(slime-repl-test-markers): More powerful macro for testing repl text.
(slime-check-buffer-contents): Simplified with
`slime-repl-test-markers'.
(repl-test): tests with no output don't expect these markers.
(repl-type-ahead): tests with no output don't expect these
markers.