lib/interval_tree: add test case for interval_tree_iter_xxx() helpers

Verify interval_tree_iter_xxx() helpers could find intersection ranges
as expected.

[sfr@canb.auug.org.au: some of tools/ uses -Wno-unused-parameter]
  Link: https://lkml.kernel.org/r/20250312113612.31ac808e@canb.auug.org.au
Link: https://lkml.kernel.org/r/20250310074938.26756-5-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>