Cleanup asyncHelpers

[gibson042]

CONTRIBUTING.md

• Only mention $DONE after asyncTest.
• Document that asyncTest is not intended to be called multiple times per file.

harness/asyncHelpers.js

• Add doc comments.
• Refactor assert.throwsAsync to fail fast and isolate error conditions.
• Clean up assert.throwsAsync failure messages (remove mention of an "inner" promise/thenable).
• Reduce the size of assert.throwsAsync for better comprehensibility in one screenful.

[ptomato]

Thanks for the clear split into commits, that was very helpful when reviewing.

The only thing I have reservations about is renaming innerThenable to p in the final commit — I don't think that improves the readability. Everything else looks good.