ship refactor followup: unit test for pre-leap3/4 ship logs

[spoonincode]

Leap4 modified the ship log format in a non-backward compatible way. That is, once Leap4+ (or Spring1+) writes to a ship log, it is no longer readable with with Leap3. Furthermore, Leap3 modified the ship log such that it is not backward compatible with any versions of eosio.

However, Spring1 is expected to read all 3 log entry formats (eosio, leap3, leap4+). There needs to be some tests that manually craft at least a leap3 log and ensure spring1 reads it properly. I'm less concerned about the eosio format since leap3 is so similar to the eosio format, but it'd be a nice to have.