[Intel-gfx] Provide a simulation friendly test environment v2

[Damien Lespiau]

Second installment of this series.

Patches 1-6 are trivial general cleanups I came across while doing this,
patches 7-10 are the real thing.

The changes from the last version are:
  • Move from a white list of tests to run to a black list of tests to skip
  • Name the environment variable that controls the simulation behaviour to
    INTEL_SIMULATION to have a chance to share the name with the piglit tests.

-- 
Damien