lib: Move KUnit tests into tests/ subdirectory

Following from the recent KUnit file naming discussion[1], move all
KUnit tests in lib/ into lib/tests/.

Link: https://lore.kernel.org/lkml/20240720165441.it.320-kees@kernel.org/

 [1]
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: David Gow <davidgow@google.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Rae Moar <rmoar@google.com>
Link: https://lore.kernel.org/r/20241202075545.3648096-4-davidgow@google.com


Signed-off-by: Kees Cook <kees@kernel.org>