tests: skip system tests when firefox isn't installed.

test/application_system_test_case.rb

@@ -3,4 +3,8 @@ require "test_helper"
 class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
   include SystemTestHelper
   driven_by :selenium, using: :headless_firefox, screen_size: [1400, 1400]
+
+  setup do
+    skip "Firefox not installed" unless system("firefox --version > /dev/null")
+  end
 end