lib/rbtree: split tests

Current tests are gathered in one big function.

Split tests into its own function for better understanding and also it
is a preparation for introducing new test cases.

Link: https://lkml.kernel.org/r/20250310074938.26756-3-richard.weiyang@gmail.com


Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michel Lespinasse <michel@lespinasse.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>