drm/amdgpu: fix preemption unit test

Remove signaled jobs from job list and ensure the
job was indeed preempted.

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>