Tag some unstable tests, and skip by default (bug#24503)

Tag some unstable tests, and skip by default (bug#24503)

* Makefile.in (check-all): New phony target.
* test/Makefile.in (SELECTOR_DEFAULT, SELECTOR_EXPENSIVE):
Also skip unstable tests.
(SELECTOR_ALL): New variable.
(check-all): New phony target.
* test/lisp/emacs-lisp/eieio-tests/eieio-test-methodinvoke.el
(eieio-test-method-order-list-6):
* test/lisp/emacs-lisp/eieio-tests/eieio-tests.el
(eieio-test-37-obsolete-name-in-constructor):
Mark as unstable rather than skipping on hydra.nixos.org.