drm/i915/selftests: Check timeout before flush and cond checks

Allow a bit of leniency for the CPU scheduler to be distracted while we
flush the tasklet and so ensure that we always check the status of the
request once more before timing out.

v2: Wait until the HW acked the submit, and we do any secondary actions
for the submit (e.g. timeslices)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200330121644.25277-1-chris@chris-wilson.co.uk