tests: Anonymous mmap microbenchmark

This adds a simple mmap and write microbenchmark that can be run on both
OSv and Linux.  The same test is run for both demand paged and
pre-populated memory region using MAP_POPULATE flag with mmap().

The numbers on 2-core Sandy Bridge i7 are as follows in an OSv guest:

       time (seconds)
   MiB demand populate
     1 0.001  0.000
     2 0.000  0.000
     4 0.000  0.000
     8 0.001  0.000
    16 0.003  0.000
    32 0.007  0.000
    64 0.012  0.000
   128 0.046  0.001
   256 0.091  0.001
   512 0.187  0.003
  1024 0.374  0.005

For reference, running the same test on Linux host gives:

       time (seconds)
   MiB demand populate
     1 0.000  0.000
     2 0.001  0.000
     4 0.001  0.000
     8 0.002  0.000
    16 0.003  0.000
    32 0.009  0.000
    64 0.021  0.000
   128 0.039  0.001
   256 0.077  0.001
   512 0.150  0.002
  1024 0.300  0.005

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
tests: mincore() tests for demand paging

As suggested by Nadav, add tests for mincore() interraction with demand
paging.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>