When you ran emacs ~/robe/core-tests.el, that opened up the file in your Emacs, right?
Then the tests themselves have to be loaded into Emacs. The ert tests are written in ELisp, so you need to evaluate that file. That can be done with M-x eval-buffer, or if the file is not open, you can run M-x load-file.
Then M-x ert will run those tests. Simply opening the file is not enough.