mm/hmm/test: add selftests for HMM

Add some basic stand alone self tests for HMM.
The test program and shell scripts use the test_hmm.ko driver to exercise
HMM functionality in the kernel.

Link: https://lore.kernel.org/r/20200422195028.3684-3-rcampbell@nvidia.com


Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>