Make number of Miller-Rabin tests for a prime tests depend on the security level of the prime

The old numbers where all generated for an 80 bit security level. But
the number should depend on security level you want to reach. For bigger
primes we want a higher security level and so need to do more tests.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
GH: #6075
Fixes: #6012