Add --no-pause flag to wpt run

[jeffcarp]

This might speed up our runs. Brought up by @JKereliuk

[kereliuk]

I'm not sure if this makes a difference for runs with a lot of tests as opposed to single run tests, I'm going to look at what it actually does under the hood though....

[jgraham]

It means that it won't pause until the browser is closed after each test. You should supply it because by default we'll pause if we are only running one tests (common debugging behaviour, uncommon on CI), but it won't make any performance difference.