test: coverage for ipex exn recipient endpoint for multi-sig IPEX

[iFergal]

Coverage for #286

@lenkan Since these are unit tests, they don't rely on real exn messages being passed back and forth over HTTP. The messages are just cloned on the other agent, which is why the tests didn't catch it.

Added some assertions for where the messages would be sent on a real deployment.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.20%. Comparing base (18d3ad7) to head (fee687a).
Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #288      +/-   ##
==========================================
+ Coverage   93.06%   93.20%   +0.13%     
==========================================
  Files          36       36              
  Lines        7121     7766     +645     
==========================================
+ Hits         6627     7238     +611     
- Misses        494      528      +34     

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[iFergal]

By the way @lenkan there's 2 ways to do multi-sig IPEX.

e.g. for admit

• Create IPEX admit and call submitAdmit
• Create /multisig/exn with admit embedded and send exn to group members individually with sendFromEvents

OR

• Create IPEX admit but don't send
• Create /multisig/exn with admit embedded and call submitAdmit with the /multisig/exn

This PR tackles the second, more efficient route but the Signify-TS tests all seem to use the first method. Not sure what you are using on your end. Might be why no one has spotted this bug until now

[lenkan]

By the way @lenkan there's 2 ways to do multi-sig IPEX.

Thanks for the hint. I have been using the first approach (as that is what the Signify-TS examples were using).

[2byrds]

LGTM