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>

MAINTAINERS

@@ -25734,6 +25734,7 @@ F:	Documentation/core-api/xarray.rst
 F:	include/linux/idr.h
 F:	include/linux/xarray.h
 F:	lib/idr.c
+F:	lib/test_xarray.c
 F:	lib/xarray.c
 F:	tools/testing/radix-tree