drm/i915/selftests: Check for has-reset before testing hostile contexts

In order to kill off a hostile context, we need to be able to reset the
GPU. So check that is supported prior to beginning the test.

Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200402205839.25065-1-chris@chris-wilson.co.uk