exectest.py: Merge Test and ExecTest into a single class

Rather than have 2 base classes, let's just have one.

v2: - remove test that no longer is valid
v3: - fix igt using ExecTest instead of Test
    - fix Test setting two different run methods

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