drm/i915/selftests: wait for active idle event in i915_active_unlock_wait

After i915_active_unlock_wait i915_active can be still non-idle due
to barrier async handling in signal_irq_work. As a result one can observe
following errors:
bcs0: heartbeat pulse did not flush idle tasks
*ERROR* pulse active pulse_active [i915]:pulse_retire [i915]
*ERROR* pulse    count: 0
*ERROR* pulse    preallocated barriers? no

To prevent it let's wait explicitly for idleness.

v2: wait only in live_idle tests

Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231205-selftest_wait_for_active_idle_event-v2-1-1437d0bf9829@intel.com