SI-7349 Partest supports test-interface

This commit updates Scalacheck to 1.10.1 and invokes it using the
test-interface API.  This obviates kludgy output checking for result
status.

When reviewing, @paulp could help me by confirming that the "test writers" in Runner aren't intended to be used anymore by Testicolor. Setting them null didn't break anything.

In fastest, I kept the LogContext entity as a stack of outputs, the equivalent of a transcript -- actually pairing std outputs and logs (and distinguishing between them) -- so my conceptual model is skewed.

This PR should be rebased after https://github.com/scala/scala/pull/2385 which is a babier step.