benchtests: Improve reliability of memcmp benchmarks

No bug. Remove reallocation of bufs between implementation tests. Move
initialization outside of foreach implementation test loop. Increase
iteration count.

Generally before this commit was seeing a great deal of variability
between runs. The goal of this commit is to make the results more
reliable.

Benchtests build and bench-memcmp succeeding.

Signed-off-by: Noah Goldstein <goldstein.w.n@gmail.com>