Improve `IOutputsMerkleRootValidator` tests in `DaveConsensus`

[GCdePaula]

• Add test cases where Merkle proof fails for each possible reason;
• Test exactly the TX buffer leaf value, instead of using an address space with repeated states;
• Create function that generates siblings from random seed---instead of fuzzing of the output tree (and repeating it to generate an address space), let the siblings be fuzzed;
• Check that outputs Merkle root is invalid before settlement and valid after settlement.