Include testcases for constructing the HTLCs in the RFCs

[thomaseizinger]

In order to build a 2nd implementation of COMIT that is compatible with the reference implementation, we specify the HTLCs as HEX codes in the RFCs and document the byte ranges which have to be replaced with the respective parameters.

• https://github.com/comit-network/RFCs/blob/master/RFC-005-SWAP-Basic-Bitcoin.md#contract
• https://github.com/comit-network/RFCs/blob/master/RFC-007-SWAP-Basic-Ether.md#contract
• https://github.com/comit-network/RFCs/blob/master/RFC-009-SWAP-Basic-ERC20.md#contract

In order to make sure that OUR implementation does the correct thing and to allow other implementors to have the same confidence, we should add test cases to all of these RFCs that can be taken and directly converted to unit tests:

Given the HTLC hex and concrete parameters, when the HTLC is constructed, should yield the correct hex.

Our test cases will need to provide:

• the HTLC hex
• concrete parameters
• the correct hex

[thomaseizinger]

In addition to testcases for generating the correct HTLC, I think we should also include test cases for how to trigger the redeem and refund part.

For Bitcoin this is basically the witness stack that needs to be provided.
For Ethereum, it boils down to the data sent to the contract with the correct gas amounts.

Maybe the testcases should be provided on a Transaction basis? i.e. fully signed and valid transactions?

/cc @bonomat @D4nte