New convenience macros ert-with-temp-(file|directory)

New convenience macros ert-with-temp-(file|directory)

* lisp/emacs-lisp/ert-x.el
(ert-with-temp-file)
(ert-with-temp-directory): New macros.
* test/lisp/emacs-lisp/ert-x-tests.el
(ert-x-tests-with-temp-directory)
ert-x-tests-with-temp-directory/text-signals-error
(ert-x-tests-with-temp-file)
(ert-x-tests-with-temp-file/handle-error)
(ert-x-tests-with-temp-file/prefix-and-suffix-kwarg)
(ert-x-tests-with-temp-file/text-kwarg)
(ert-x-tests-with-temp-file/unknown-kwargs-signals-error): New tests.