drm/i915/selftests: Improve error detection of reset failure

Use a timedwait to promptly detect if the recovery after reset fails and
provide a meaningful debug dump.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190312111146.10662-2-chris@chris-wilson.co.uk