Increase browser start timeout.

30s seems to be a bit too slow in some circumstances with GitHub Actions
CI.

Author: Robert Jackson

testem.js

@@ -3,6 +3,7 @@ module.exports = {
   disable_watching: true,
   launch_in_ci: ['Chrome'],
   launch_in_dev: ['Chrome'],
+  browser_start_timeout: 120,
   browser_args: {
     Chrome: {
       ci: [