Use the same root test class.

Instead of being strongly tied to unittest2 make
it easier for taskflow to switch to another root
class (testr?) by abstracting out the unittest2
usage. This also enables some useful functionality
to exist in that root test class that other tasks
can take advantage of.

Change-Id: I381b6fb07e47f984b44cde439a17f39a1c1d32ac