drm/i915/selftests: Hide dangerous tests

drm/i915/selftests: Hide dangerous tests

Some tests are designed to exercise the limits of the HW and may trigger
unintended side-effects making the machine unusable. This should not be
executed by default, but are still useful for early platform validation.

References: https://bugs.freedesktop.org/show_bug.cgi?id=103453
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171025153207.9589-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>