firewire: add KUnit test to check layout of UAPI structures

In future commits, some new structure will be added to express new type
of event. They are exposed to user space as the part of UAPI. It is likely
to get trouble in ioctl compatibility layer for 32 bit binaries in 64 bit
host machine since the layout of structure could differ depending on
System V ABI for these architectures. Actually the subsystem already got
such trouble at v2.6.27. It is preferable to decide the layout of
structure carefully so that the layer is free from such trouble.

This commit utilizes KUnit framework to check the layout of structure for
the purpose. A test is added for the existent issue.

Cc: kunit-dev@googlegroups.com
Link: https://lore.kernel.org/r/20230529113406.986289-2-o-takashi@sakamocchi.jp


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