zerochecksum: More comprehensive testing, test fix
zerochecksum: More comprehensive testing, test fix

Test the various scenarios of two nodes having or not having
zero-checksums enabled.

Also fixing a (non-triggered) error in the test case which was
setting
   conn.allow0Checksum = True

which doesn't do anything as the conn is the callback but not
the actual mininode.

BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited