Test Emacs version on GitHub Actions

## What I did
Added the Emacs ERT tests to the GitHub Actions workflow, after TravisCI stopped working.

## What happened
It complained with this error:

```
emacs: standard input is not a tty
```

## What I wanted to happen
Run the way it used to on Travis.

## Analysis
I found a bunch of actions to help:
* [setup emacs](https://github.com/marketplace/actions/set-up-emacs)
* [emacs lisp check](https://github.com/marketplace/actions/emacs-lisp-check) I haven't tried it yet, but it looks like it can run ERT.
* [cask](https://github.com/cask/cask) I used it on Travis, but I think it's not needed with setup emacs.
* There's some discussion on actions/runner#241. That's where I learned about `script` and `scriptreplay`.

Start by reading the GitHub Actions workflow and the existing TravisCI setup for the Emacs ERT tests. Reproduce the `emacs: standard input is not a tty` failure, then confirm that the workflow runs the ERT tests successfully without requiring a terminal.

Written by the indexing model from the issue text.