tests/gem_dummy_reloc_loop: Add one subtest based on multi drm_fd to test CPU<->GPU...

tests/gem_dummy_reloc_loop: Add one subtest based on multi drm_fd to test CPU<->GPU 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 CPU<->GPU sync for the second BSD ring.

V1->V2: Follow Daniel's comment to add one subtext instead of one individual
test case, which is used to test the CPU<->GPU sync under multi BSD rings.

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

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>