Silence pos/t3960's -Ycheck output.

Someday someone will have to straighten out where output goes.
Clearly under current conditions, Console.err is not a good place.
I rerouted through unit.warning so the output will be swallowed
like all the other warnings.
Merge pull request #2960 from paulp/noisy-test-3960

Silence pos/t3960's -Ycheck output.