MAINTAINERS: clarify maintenance of nvdimm testing tool

The git history shows that the files under ./tools/testing/nvdimm are
being developed and maintained by the LIBNVDIMM maintainers.

This was identified with a small script that finds all files only
belonging to "THE REST" according to the current MAINTAINERS file, and I
acted upon its output.

Signed-off-by: Lukas Bulwahn <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dan Williams <[e-mail]>

MAINTAINERS

···
 F:	include/linux/nd.h
 F:	include/linux/libnvdimm.h
 F:	include/uapi/linux/ndctl.h
+F:	tools/testing/nvdimm/
 
 LICENSES and SPDX stuff
 M:	Thomas Gleixner <[e-mail]>