Evaluate benefits of marble testing

[j2L4e]

See https://github.com/ReactiveX/rxjs/blob/master/doc/marble-testing.md

The current tests are hard to reason about and some use hacky setTimeouts just to make it work.

This could possibly be improved by using marbles.

[stale]

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[daffl]

Oops... forgot to add the label to keep this open.