add a `waitUntil()` style function for tests

[thedavidmeister]

Lots of flaky tests happening at the moment.

e.g. https://github.com/holochain/holochain-proto/pull/683

Typically takes 3-4 tries to get tests through Travis.

One simple approach that might help things is to standardise a way to wait until a predicate is satisfied rather than waiting an exact time.

[zippy]

Nice, yes we should definitely clean things up with this pattern.

[thedavidmeister]

i'm doing some googling to see if there's a way to directly observe value changes of variables with events/callbacks in go

ideal: when(pred, timeout) maybe OK: waitUntil(pred, timeout) will frequently break: waitFor(pred, ms)

[thedavidmeister]

so as per https://github.com/holochain/holochain-proto/pull/689/files when could look like when(pass-pred, error-pred, chan)