MAINTAINERS: add lib/test_xarray.c

Ensure test-only changes are sent to the relevant maintainer.

Link: https://lkml.kernel.org/r/20250129-xarray-test-maintainer-v1-1-482e31f30f47@gmail.com


Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Cc: Mattew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>