* emacs-lisp/ert.el (ert-deftest): Bind macro `skip-unless'.

* emacs-lisp/ert.el (ert-deftest): Bind macro `skip-unless'.
(ert-test-skipped): New error.
(ert-skip, ert-stats-skipped): New defuns.
(ert--skip-unless): New macro.
(ert-test-skipped): New struct.
(ert--run-test-debugger, ert-test-result-type-p)
(ert-test-result-expected-p, ert--stats, ert-stats-completed)
(ert--stats-set-test-and-result, ert-char-for-test-result)
(ert-string-for-test-result, ert-run-tests-batch)
(ert--results-update-ewoc-hf, ert-run-tests-interactively): Handle
skipped tests.