drm/i915/selftests: Relax timeout for error-interrupt reset processing

We can not require that the system process a tasklet in reasonable time
(thanks be to ksoftirqd), but we can insist that having waited
sufficiently for the error interrupt to have been raised and having
kicked the tasklet, the reset has begun and the request will be marked
as in error (if not already completed).

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/20200210205722.794180-3-chris@chris-wilson.co.uk