tests: mem_protect/demand_paging: wait a bit for NRU to work

This waits a bit for NRU eviction algorithm (which is the default)
to work its magic to clear the access bit of physical frames.
This increases the number of clean pages which can be evicted,
to make sure the number of clean pages evicted is not zero, which
would cause an assertion.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>