test_maple_tree: add test for mas_spanning_rebalance() on insufficient data

Add a test to the maple tree test suite for the spanning rebalance
insufficient node issue does not go undetected again.

Link: https://lkml.kernel.org/r/20221219161922.2708732-3-Liam.Howlett@oracle.com
Fixes: 54a611b6

 ("Maple Tree: add new data structure")
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Andrei Vagin <avagin@gmail.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>