test/amdgpu/hotunplug: Add test suite for GPU unplug

Add plug/unplug device and open/close device file
infrastructure.
Add basic test - unplug device while device file still
open. Close device file afterwards and replug the device.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>