Add a test for page fault handling.

This both tests that they work correctly and prints timing information
if you run it manually.
Add a define to use sigaction on OS X instead of OS-specific page fault handling.

The latter was introduced last year in order to allow debugging without
turning off Fastmem; I wanted to see if the old way was faster.  In
fact, it is not, but may as well keep the define around in case future
testing is necessary.
Merge pull request #1344 from comex/mem-tools-testing

MemTools testing