[Intel-gfx] Craft a list of tests to run in simulation (and tune them)

[Damien Lespiau]

For early bring-up, it's useful to have a list of tests to check everything is
in order. The Makefile file is a great place to keep such a list.

The IGT_SIMULATION environment variable controls the list of tests to run
(either with make test/check or with piglit) and allows us to tune the various
tests we execute to run in an acceptable amount of time in simulation.