refactor(heart): bind class methods and make beat async (#5142)

* feat: set up new test for beat twice

* refactor: make Heart.beat() async

This allows us to properly await heart.beat() in our tests and remove
the HACK I added before.

* refactor: bind heart methods .beat and .alive

This allows the functions to maintain access to the Heart instance (or
`this`) even when they are passed to other functions. We do this because
we pass both `isActive` and `beat` to `heartbeatTimer`.

* feat(heart): add test to ensure no warnings called

* fixup!: revert setTimeout for heartbeatTimer

* fixup!: return promise in beat