[dotest] Consider unexpected passes as failures.

Unexpected successes should be considered failures because they can hide
regressions when not addressed. When a test is fixed and not re-enabled,
it can easily regress without us noticing.

I couldn't find a good way to make this change other than changing it in
the unittest2 framework. I know this is less than optimal but since we
have the dependency checked in and the change is pretty fundamental to
the framework I think it's not unreasonable.

Differential revision: https://reviews.llvm.org/D55835

llvm-svn: 349818