Provide regression tests for submitting contesting proofs vol. 2

[sdaveas]

Issue #9 did some high level testing but there are more some case to be addressed.
This will close #2 as well

Check if nipopowContract.sol works correctly under in the following cases:

1. The interlinks are invalid
   1. Inclusion fail (wrong Merkle Tree) (see Provide regression tests for submitting contesting proofs vol. 2 #13 (comment))
   2. Block(s) indicated by some interlink at some level is missing (see Provide regression tests for submitting contesting proofs vol. 2 #13 (comment))
   3. Leaf of Merkle Tree does not point to the previous block
2. Proof succeeds at all cases regarding different μ s
   1. μ1 = μ2
   2. μ1 < μ2
   3. μ1 > μ2
3. Chain does not start from genesis (see Provide regression tests for submitting contesting proofs vol. 2 #13 (comment))
4. Less collateral than needed is provided (see (Provide regression tests for submitting contesting proofs vol. 2 #13 (comment)))
5. Check if each block has the difficulty of genesis target
   1. Contract should fail calling submit with insufficient funds
   2. Refund collateral to honest caller

[sdaveas]

Case 3 fails. Contract needs to know hash of the genesis block. We have to provide that upon instantiation of is there any way to avoid that? Client should be able use the contract for different blockchains as well.

[sdaveas]

Case 1i and 1ii works, see 32aa848

[sdaveas]

Case 4 works, see 837069a