Small primes are primes too.

Previously, BN_is_prime_fasttest_ex, when doing trial-division, would
check whether the candidate is a multiple of a number of small primes
and, if so, reject it. However, three is a multiple of three yet is
still a prime number.

This change accepts small primes as prime when doing trial-division.

Reviewed-by: Kurt Roeckx <kurt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3264)