rename test helpers shouldProduceError/shouldNotProduceError to shouldReject/shouldResolve, resp.
add comments on test helpers shouldReject and shouldResolve
have failure msgs of testhelpers should[Reject|Resolve] contain "reject"/"resolve"; more selective use of hint about possible misuse; tests require only at least 1 fail, and only assert on the message of the first
improve tests for test helpers should[Reject|Resolve]: more tests with less assertions each
improve comments on test helpers should[Reject|Resolve]