I initially tried to write a WPT test for this. But I failed to do so
because the test driver for WPT doesn't accept a pointerdown without any
matching pointer events. So either it just doesn't fire a pointerdown or
it fires pointerup with it. So that's why I chose to write one in
mochitest instead.
The test first fires a pointerdown with no up, and then fires another
pointerdown with an up instead. So the second pointerdown should clear
the old one and set its interactionId to zero. So first one should have
interactionId of zero and the second should have a non-zero
interactionId.
Differential Revision: https://phabricator.services.mozilla.com/D252110
