Vet + update WebPageTest's Time to Interactive (TTI) metric (for Firefox, at least)

The royal "We" should investigate and vet the current measurement of Time to Interactive in the WebPageTest core/server: https://github.com/WPO-Foundation/webpagetest/blob/79f638047103cf857dae24116c93b11d027e1bab/www/page_data.inc#L265

Especially in light of Randell landing https://bugzilla.mozilla.org/show_bug.cgi?id=1299118 along with some Long Tasks support: the closer we get to implementing the "spec" (draft? proposed spec?), the closer we should be to how Chrome and other browsers are measured/measuring it.