lib/tests/kfifo_kunit.c: add tests for the kfifo structure

lib/tests/kfifo_kunit.c: add tests for the kfifo structure

Add KUnit tests for the kfifo data structure.
They test the vast majority of macros defined in the kfifo
header (include/linux/kfifo.h).

These are inspired by the existing tests for the doubly
linked list in lib/tests/list-test.c (previously at lib/list-test.c) [1].

Note that this patch depends on the patch that moves the KUnit tests on
lib/ into lib/tests/ [2].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/lib/list-test.c?h=v6.11-rc6
[2] https://lore.kernel.org/all/20240720181025.work.002-kees@kernel.org/

Signed-off-by: Diego Vieira <diego.daniel.professional@gmail.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Rae Moar <rmoar@google.com>
Link: https://lore.kernel.org/r/20241202075545.3648096-5-davidgow@google.com
Signed-off-by: Kees Cook <kees@kernel.org>