When you ran `emacs ~/robe/core-tests.el`, that opened up the file in your Emacs, right?

But the tests themselves have to be loaded into Emacs. The `ert` tests are written in ELisp, so you just need to evaluate that file. You can do that with <kbd>M-x eval-buffer</kbd>, or if it's not open, you can run <kbd>M-x load-file</kbd>.

Then <kbd>M-x ert</kbd> will run those tests. Simply opening the file is not enough.