tests/gpu.py: Don't use execfile

This uses import rather than exefile. It requires a few additional
changes since gpu.py uses some hacky behavior to remove shader_tests

XXX: This isn't ready to be pushed
tests/r300.py: use import instead of execfile

Signed-off-by: Dylan Baker <baker.dylan.c@gmail.com>
tests/r500.py use import instead of execfile

Signed-off-by: Dylan Baker <baker.dylan.c@gmail.com>
core.py: rewrie loadTestProfile to remove execfile()

This patch removes execfile, replaceing it with import instead.

This isn't a prefect solution, but it provides feature parity with the
execfile() aproach, while being python3 safe.