Add tests on the messages exchanged when peers start replicating a core

[HDegroote]

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[HDegroote]

The non-sparse core might be oversending sync messages. It sends the same sync message 3 times:

 {
  fork: 0,
  length: 5,
  remoteLength: 0,
  canUpgrade: true,
  uploading: true,
  downloading: true,
  hasManifest: true
}

The empty cores first sends

{
  fork: 0,
  length: 0,
  remoteLength: 0,
  canUpgrade: true,
  uploading: true,
  downloading: true,
  hasManifest: false
}

It then sends a second message with updated remoteLength, and a third where both remoteLength and length are set to 5. That seems ok