Test payjoin_ffi backwards compatibility with v1

[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.

[spacebear21]

v2-to-v2 is now covered.

v1-to-v2 and v2-to-v1 are missing from payjoin-ffi integration tests.

[benalleng]

I have started building out v2_to_v1 it's almost there, I just have 2 functions to bring over the ffi layer