The only way to kill `yarn test --watch` is sending a SIGINT via a
<kbd>Ctrl</kbd>+<kbd>C</kbd> keyboard shorcut. Killing karma process in
this way sometimes leaves the parent gulp process orphan. That's because
when karma is killed by SIGINT sometimes doesn't run the `done` in the
callback, hence leaving the gulp process waiting for the `done` signal.

On more rare occasions, I have seen orphan karma process too.

The solution presented here registers a listener for SIGINT and call the
`done` function. It doesn't unregister the event.