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 M-x eval-buffer, or if it's not open, you can run M-x load-file.
Then M-x ert will run those tests. Simply opening the file is not enough.