Add editor unit test using `expect`

[jyn514]

The editor function is currently untested for most scenarios outside of a linux machine. The command expect can access the X-server API to ensure the correct response is given to a command.

[charlesdaniels]

The editor function is currently untested for most scenarios outside of a linux machine. The command expect can access the X-server API to ensure the correct response is given to a command.

`expect` actually just works with stdin/stdout/stderr, no need to spin up an X-Server (which would make our tests take forever). I don’t think it’s necessary to test the editor read function with every possible editor the user might want to use, I think just testing one CLI editor should be adequate to ensure the functionality is working. Maybe two if we’re feeling ambitious. In theory, it would be a good idea to test using this feature with GUI editors on Windows, but I don’t feel like figuring AppVeyor out just for that. If we ever did want to do actual GUI testing (weather with X11, Wayland, or Windows), the tool to use would be Sikuli X, but I really think that would be totally overkill for a project this small.