[Piglit] Radeon specific tests

[Tom Stellard]

The second patch in this series adds two tests for a radeon specific
(although maybe other hardware has something similar, I'm not sure)
hardware feature known as presubtract, which does "free" math operations
on sources before they are read by an instruction.  I added these tests
to radeon.tests instead of all.tests, because I don't want to spam the
other drivers with tests that aren't relevant to them.  Is this the
preferred way to handle this, or should everything go into all.tests?

-Tom