firewire: test: add KUnit test for device attributes

The traverse over CSR space results in attributes of node and unit
devices. Any test of the traverse is useful.

This commit adds a skeleton of KUnit test for the purpose.

Link: https://lore.kernel.org/r/20231221134849.603857-4-o-takashi@sakamocchi.jp


Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>