Update WPTs to not be kind-agnostic

[henbos]

The current tests are happy with either video or audio metrics, but some are only defined for one of the kinds.

Having WPTs have separate tests for different kinds would help with #660, where we update the spec to say which kinds are defined for a metric.

[fippo]

this mainly affects https://github.com/web-platform-tests/wpt/blob/master/webrtc-stats/supported-stats.html ?

[henbos]

Yes!