tools/testing/nvdimm: mock ioremap_wt

In the 4.2-rc1 merge the default_memremap_pmem() implementation switched
from ioremap_nocache() to ioremap_wt().  Add it to the list of mocked
routines to restore the ability to run the unit tests.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>