This used to be ERT, a tool for automated testing in Emacs Lisp.

ERT is now included in Emacs trunk; this repository is no longer
maintained.  To get ERT, either build Emacs from trunk, or grab
the following files which represent the last state of ERT before
its installation into Emacs:

https://github.com/ohler/ert/blob/c619b56c5bc6a866e33787489545b87d79973205/lisp/emacs-lisp/ert.el
https://github.com/ohler/ert/blob/75ca9dd0859d16db4da88eec0c15fbc43f25ca97/lisp/emacs-lisp/ert-x.el
https://github.com/ohler/ert/blob/bc51e3e8121d51dc88618847f0958e7ec832c7d2/test/automated/ert-tests.el
https://github.com/ohler/ert/blob/75ca9dd0859d16db4da88eec0c15fbc43f25ca97/test/automated/ert-x-tests.el