wmem: get rid of the allocator timing test

Some of the allocators are close enough that a little bit of system load at the
time of the test was enough to throw them off, and it was causing unnecessary
buildbot failures. Benchmarking on real-world cases at the time of
implementation (as well as reliable "average" results on the buildbots) has left
me confident that block_fast is, in fact, faster than block, which is faster
than simple/strict.

Additionally, the allocators are effectively "done" and unlikely to change, so
the possibility of uncaught performance regressions is low.

Change-Id: I144ea4cd9d6000edc4075ed682ce6a4aeaf5e28a
Reviewed-on: https://code.wireshark.org/review/3356
Reviewed-by: Anders Broman <a.broman58@gmail.com>