* ci: run linux arm tests on CircleCI

* cleanup electron dirs after testing

(cherry picked from commit 1c0a6045fbae8f9111fdd92730e5a81652e5a911)

* use start-stop-daemon to kill Xvfb

(cherry picked from commit 1d10a68c31eda061f3804d4f08d91e77f2647ab8)