Clarigen tests for xbtc ft/nft swaps

[mijoco-btc]

Basic set of tests using Clarigen and instructions for setting it up in README.

One test raised a question about reclaiming the primary in the case fee contract has been compromised. See "Can cancel after expiry and get primary back even if the fee contract cant return the fees for any reason".

Probably out of scope for this repo - happy to comment out the test!