We can build Fitness again now that playfair has been updated to account for
the package:flutter rename.

Also includes changes to widget_tester sufficient to
test scrolling.

@abarth

This patch adds a new test harness and a first, trivial test to run with the
harness. The new test harness is built on package:test and should run on
Travis. Over time, we'll migrate our existing tests into this harness.