test_xarray: add tests for advanced multi-index use

The multi index selftests are great but they don't replicate
how we deal with the page cache exactly, which makes it a bit
hard to follow as the page cache uses the advanced API.

Add tests which use the advanced API, mimicking what we do in the
page cache, while at it, extend the example to do what is needed for
min order support.

Tested-by: Daniel Gomez <da.gomez@samsung.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
1 file changed