Fix KeyPairGenerator test for DH

Finding safe primes for the DH key pair generator can take up to 20
minutes sometimes. Use a pre-defined set of primes instead of skipping
the test altogether because of the extreme amount of time it takes.

This leaves us not testing one software path, but this situation is
strictly better than not running the test at all.

Change-Id: I03d457747e2ec1a2592c1c6350365cbe5aa33759