MAINTAINERS: clarify maintenance of nvdimm testing tool

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 <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20200201170933.924-1-lukas.bulwahn@gmail.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>