Split off stat (benchmark) test failures into a separate section in the test runner summary.

Stat tests are generally less reliable than other types of tests, so it's nice to have
them in a separate section rather than interspersed with potential...

Summary: ...correctness issues.

Reviewers: austin

Reviewed By: austin

Subscribers: thomie, carter, simonmar

Differential Revision: https://phabricator.haskell.org/D406