[Piglit] More exectest cleanups and refactors

[Dylan Baker]

This series offers additional cleanup, re-factors and optimizations to
the Test class. This series puts a lot of work into Test.run() and
Test.execute(), with a focus on reducing overall code, specifically
function calls, combining if trees, and replacing slow with generators
or comprehensions.

Patches 1-2 lay groundwork for making these changes
Patches 3-13 make incremental cleanups to Test.