Don't overtest: speed up contrib-specific tests.

* Makefile ($(CONTRIBS:%=check-%) check): Speed up target, set
SELECTOR for for these targets to '(tag contrib).

* slime-tests.el (slime-test-ert-test-for): Use
`define-slime-ert-test'.
(slime-tests-auto-tags): Set ert tags automatically.
(define-slime-ert-test): Use `slime-tests-auto-tags'.
No more Emacs 23 gotchas.

* README.md (Emacs 23 gotchas): Remove section.