python/crypto/dhparam_tests: use 512 bits by default
python/crypto/dhparam_tests: use 512 bits by default

On Fedora 36, OpenSSL 3.0.0 forbids using DH parameters lower than 512
bits:

    $ openssl dhparam -2 256
    Generating DH parameters, 256 bit long safe prime
    dhparam: Generating DH key parameters failed
    807B1DC55D7F0000:error:0280007E:Diffie-Hellman routines:dh_builtin_genparams:
    modulus too small:crypto/dh/dh_gen.c:168:

Increase the default bit size to 512.

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