Bump (again) the amount of work we do for wmem's timing test

Apparently the new win8 vm has a *very* low-resolution timer, and the allocators
are indistinguishable at the previous amount of work.

Change-Id: If4e5bb8f85b1f0d39658f54001c88f42ffddfc47
Reviewed-on: https://code.wireshark.org/review/2768
Reviewed-by: Evan Huus <eapache@gmail.com>