tools/testing/nvdimm: make iset cookie predictable

For testing changes to the iset cookie algorithm we need a value that is
constant from run-to-run.

Stop including dynamic data in the emulated region_offset values. Also,
pick values that sort in a different order depending on whether the
comparison is a memcmp() of two 8-byte arrays or subtraction of two
64-bit values.

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