fpga: add an initial KUnit suite for the FPGA Bridge

The suite tests the basic behaviors of the FPGA Bridge including
the functions that operate on a list of bridges.

Signed-off-by: Marco Pagani <marpagan@redhat.com>
Acked-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20230718130304.87048-3-marpagan@redhat.com
Signed-off-by: Xu Yilun <yilun.xu@intel.com>