Add invoice serialization tests [TestOnly][Minor]

[optout21]

This PR only adds some tests for invoice serialization/deserialization, to create a test harness in preparation of Bech32 encoding changes (#3244 ).
This PR can be merged optionally, or skipped, and the tests added with #3244 . It is more clear that behavior does not change if tests are added upfront.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.87%. Comparing base (0d7ae86) to head (1248139).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3266      +/-   ##
==========================================
+ Coverage   89.82%   89.87%   +0.05%     
==========================================
  Files         125      126       +1     
  Lines      102830   103436     +606     
  Branches   102830   103436     +606     
==========================================
+ Hits        92368    92965     +597     
- Misses       7752     7760       +8     
- Partials     2710     2711       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[TheBlueMatt]

Otherwise LGTM, thanks.