Switches to the headless chrome driver

This prevents the browser from stealing focus from your editor/terminal
when running integration tests.

Once in a while I'm sure it's helpful to have this jump into the
foreground but for day to day use I think it's better to have it in the
background.

test/application_system_test_case.rb

@@ -1,5 +1,5 @@
 require "test_helper"
 
 class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
-  driven_by :selenium, using: :chrome, screen_size: [1400, 1400]
+  driven_by :selenium, using: :headless_chrome, screen_size: [1400, 1400]
 end