drm/i915/selftests: Mark up preemption tests for hang detection

Use the igt_live_test framework for detecting whether an unwanted hang
occurred during test execution, and report failure if it does.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190321194031.20240-2-chris@chris-wilson.co.uk