MAINTAINERS: add lib/test_xarray.c

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

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Tamir Duberstein <[e-mail]>
Cc: Mattew Wilcox <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

MAINTAINERS

···
 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