util: Add a randomized test for the virtual memory allocator

util: Add a randomized test for the virtual memory allocator

The test pseudo-randomly makes allocations and deallocations with
the virtual memory allocator and checks that the results are
consistent. Specifically, we test that:

 * no result from the allocator overlaps an already allocated range
 * allocated memory fulfills the stated alignment requirement
 * a failed result from the allocator could not have been fulfilled
 * memory freed to the allocator can later be allocated again

v2: - fix if() in test() to actually run fill()
v3: - add c++11 build flag (Jason)
    - test the full 64-bit range (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>