Remove workaround for fixed Bug#6581 from ert

Remove workaround for fixed Bug#6581 from ert

* lisp/emacs-lisp/ert.el (ert-equal-including-properties): Make
into obsolete function alias for 'equal-including-properties'.
* test/src/editfns-tests.el (format-properties):
* test/lisp/emacs-lisp/ert-x-tests.el (ert-propertized-string)
(ert-test-run-tests-interactively-2): Don't use above obsolete
name.

(ert--explain-equal-including-properties-rec): New function.
(ert--explain-equal-including-properties): Use as an explainer for
'equal-including-properties' now that Bug#6581 is fixed.

* test/lisp/emacs-lisp/ert-tests.el
(ert-test-explain-equal-string-properties): Expand test.
(ert-test-equal-including-properties): Merge test into above
expanded test.