Adds a qa testcase testing the new "-assumevalid" option. The testcase builds
a chain that includes and invalid signature for one of the transactions and
sends that chain to three nodes:
- node0 has no -assumevalid parameter and rejects the invalid chain.
- node1 has -assumevalid set and accepts the invalid chain.
- node2 has -assumevalid set but the invalid block is not buried deep
enough to assume invalid, and so rejects the invalid chain.
