Add CLI for running elixir-mode tests.

This solves an annoying issue I was having running the tests repeatedly. As I made changes, I would have to M-x `eval-buffer` for each buffer that changed, then run the tests.

Here, `run_tests` invokes a new Emacs process in batch mode. If there are test failures, the shell script gives the user the option to re-run the tests in a regular, non-batch Emacs session.