v2.0.0

Breaking

• Added test assertions to appropriate test helpers. This could break any t.plans you have.
• Fixed some t.delay functions in the various helpers. This could affect some raf sensitive tests.

Features

• Add events.once as t.once helper for async listening to event emitters in tests.
• Add t.appendChild helper. #20 🙏 @tony-go
• vhs.only test helper is forwarded from tape.