Tests match consensus decision from Eth Magicians thread (accept duplicates, accept partial for slashable data, unordered messages, optional signing roots).
Test format modified to test multiple interchanges being imported in a row, covering cases where gaps between interchanges weren't handled correctly (see https://github.com/sigp/lighthouse/issues/1873, thanks to Adrian Sutton for finding this).
Test format modified to include an allow_partial_import field
Test format modified to include signing roots in test blocks + attestations (closes #3)
See discussion here: https://ethereum-magicians.org/t/eip-3076-validator-client-interchange-format-slashing-protection/4883
allow_partial_importfield