tests: Add one ring sync case based on multi drm_fd to test ring semaphore sync under...

tests: Add one ring sync case based on multi drm_fd to test ring semaphore sync under multi BSD rings

The Broadwell GT3 machine has two independent BSD rings in kernel driver while
it is transparent to the user-space driver. In such case it needs to check
the ring sync between the two BSD rings. At the same time it also needs to
check the sync among the second BSD ring and the other rings.

V2->V3: Follow Imre's comment to remove the unnecessary initialization and
use igt_assert_f instead of igt_assert.

V3->V4: Add gem_multi_bsd_sync_loop.c into the tests/.gitignore

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>