drm/i915/selftests: Kick timeslice until selftest yields

Keep on kicking the timeslice in case on the first retirement, it did
not stay idle. This may happen when using semaphore yields.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201222113536.3775-1-chris@chris-wilson.co.uk