Some tests (the ones which call igt_setup_runtime_pm and
igt_pm_enable_audio_runtime_pm) change default system configuration and
never restore it.

The configured runtime suspend is aggressive and may influence behaviour
of subsequent tests, so it is better to restore to previous values on test
exit.

This way system behaviour, with regards to a random sequence of executed
tests, will be more consistent from one run to another.

v2: Read failure means no runtime pm support so don't assert on it.
v3: Install exit handler before the write to close the Ctrl^C race. (Imre)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> # v2
Acked-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>