radix tree test suite: Convert iteration test to XArray

With no code left in the kernel using the multiorder radix tree, convert
the iteration test from the radix tree to the XArray.  It's unlikely to
suffer the same bug as the radix tree, but this test will prevent that
bug from ever creeping into the XArray implementation.

Signed-off-by: Matthew Wilcox <willy@infradead.org>