[Piglit] Import test profiles instead of calling execfile

[Dylan Baker]

This series reworks the way piglit loads TestProfile instances from
.tests files. The goal is to do away with calls to execfile(), which has
two primary benefits. First, its bad coding practice according to the
python community. Second, execfile is not supported in python3, and the
work around is even uglier.

It ends up reworking a number of features in core, and all of the tests
files, including changing them into .py files, which is required to to
import them.

Patches 1-5: are ground work for the series
Patches 5-12: Remove execfile() and replace it with import