Add `skip-when` macro to `ert-deftest`

Add `skip-when` macro to `ert-deftest`

This can help avoid some awkward test skip conditions.

For example, this triple negation:
    (skip-unless (not noninteractive))

Can be written as the simpler:
    (skip-when noninteractive)

* lisp/emacs-lisp/ert.el (ert-deftest): Add new 'skip-when' macro.
(ert--skip-when): New internal function.
* doc/misc/ert.texi (Tests and Their Environment): Document above
new macro.
* test/lisp/emacs-lisp/ert-tests.el (ert-test-skip-when): New test.