Test 0.21 payjoin_ffi integrations

[DanGould]

v1-to-v1 is no longer the most relevant payjoin-ffi since it doesn't support payjoin-ffi doesn't ship a pure v1 receiver. The following are the relevant code paths to test:

• v2-to-v2
• v1-to-v2
• v2-to-v1 (with rust-payjoin::v1 feature, manual extract_v1 would ignore handling the v1 receiver version error)

Testing each of these should ensure that v1-to-v1 actually works, but will never be hit with our shipped state machines.