Fix silent rebase conflict that broke tests

[valentinewallace]

429cbe1 merged a PR that renamed Offer::signing_pubkey to Offer::issuer_signing_pubkey. However, there was a silent rebase conflict and a test added as part of 1059f5f did not get the memo and used the old method name, breaking the test build.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.62%. Comparing base (429cbe1) to head (487d6a0).
Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3321      +/-   ##
==========================================
- Coverage   89.63%   89.62%   -0.01%     
==========================================
  Files         126      126              
  Lines      102564   102668     +104     
  Branches   102564   102668     +104     
==========================================
+ Hits        91937    92021      +84     
- Misses       7905     7920      +15     
- Partials     2722     2727       +5     

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