minimum_depth set to a static value

[Psycho-Pirate]

According to BOLT 2, for accept_message, the minimum_depth should be determined by the sender.

The sender:

    if channel_type includes option_zeroconf:
        MUST set minimum_depth to zero.
    otherwise:
        SHOULD set minimum_depth to a number of blocks it considers reasonable to avoid double-spending of the funding transaction.

Lnprototest sets it to 3. This causes tests to fail if minimum_depth required by an implementation is not equal to 3.