fpga: Simplify and improve fpga mgr test using deferred actions

fpga: Simplify and improve fpga mgr test using deferred actions

Use deferred actions to simplify the test suite and avoid potential memory
leaks when test cases fail. Remove unnecessary calls to
kunit_device_unregister() since kunit devices are tied to the test context
and released by a deferred action when the test is completed.

Signed-off-by: Marco Pagani <marpagan@redhat.com>
Acked-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20240725125031.308195-2-marpagan@redhat.com
Signed-off-by: Xu Yilun <yilun.xu@linux.intel.com>