The previous tests used 100 parallel workers which for EC certificates
leads to a memory usage of more than 1Mb, hence leading to an out of memory
condition in tests using the memory buffer allocator which has a pool of 1Mb.
Use 25 workers but an increased number of iterations per worker instead.
This allows to build the library + tests via `make` without
specifying `PTHREAD=1`, in which case the X.509 threading
test suite will be silently dropped.
This is analogous to the pre-existing handling of the example
application `ssl_pthread_server`, which is only build if `PTHREAD=1`
and silently dropped otherwise.