[WebIDL-tests] Test [[HasInstance]] and legacy callback interface objects (#5762)

Follows https://github.com/heycam/webidl/pull/356 and also adds general coverage for legacy callback interface objects.

View on GitHub: https://github.com/w3c/web-platform-tests/pull/5762

Received on Tuesday, 2 May 2017 20:13:40 UTC