perf: Avoid taking too long time with high thread counts

perf: Avoid taking too long time with high thread counts

We change the tests to perform the same number
of iterations regardless of the thread counts.
Although this raises the running time for small
number of threads, this avoids very long running
times for 100 or more threads.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/tools/pull/162)