Add defacto

[RByers]

Since web-platform-tests accepts "defacto" tests, perhaps we could compensate for the "hand-wavy" nature of the cancelability definition by adding a few related test cases which we expect will pass in at least 2 browsers? Eg verify:

1. touchmoves are uncancelable after a scroll has started
2. On repeated fling, the first touchstart is cancelable but the following ones are not.
3. Fling of a scrollable div followed by touchstart on a sibling element results in a cancelable touchstart (while still seeing scroll events go to the scrollable div).

@NavidZ @dtapuska WDYT?