Fix Nightwatch commands in waitUntil callback return NightwatchAPI instead of a Promise

[capGoblin]

Fixes #4158
The problem is that in waitUntil if we pass an async callback to waitUntil command, Nightwatch commands inside the callback still just return NightwatchAPI and no promise which can be awaited to get the result. While in perform command, it works fine and Nightwatch commands return a promise inside async callback passed to perfom command, which this is because the alwaysAsync is set true for perform which inturn returns the promise in this code block. Similarly now waitUntil returns promise instead of the NightwatchAPI as expected. Would love to hear any suggestions/opinions.

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[gravityvi]

Can we write a test for this?