python: Rename PlainExecTest to PiglitTest

Since PlainExecTest is either used directly or as a parent for running
"native" piglit tests, it seems fitting to give it a name that reflects
that purpose.

This patch was mostly generated with the following command:
find . -name '*.py' | xargs sed -i -e 's!PlainExecTest!PiglitTest!g

Then a handful of related changes were made manually (These were mostly
docstrings or framework unit tests related)

Signed-off-by: Dylan Baker <baker.dylan.c@gmail.com>