feat(SPV-1337): paymail capabilities test

[chris-4chain]

Pull Request Checklist

• 📖 I created my PR using provided : CODE_STANDARDS
• 📖 I have read the short Code of Conduct: CODE_OF_CONDUCT
• 🏠 I tested my changes locally.
• ✅ I have provided tests for my changes.
• 📝 I have used conventional commits.
• 📗 I have updated any related documentation.
• 💾 PR was issued based on the Github or Jira issue.

[github-actions]

Manual Tests

💚 Manual testing by @dorzepowski resulted in success.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 38.59649% with 35 lines in your changes missing coverage. Please review.

Project coverage is 46.25%. Comparing base (e776a4b) to head (ad51598).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
engine/tester/jsonrequire/getter.go	0.00%	14 Missing ⚠️
engine/tester/jsonrequire/funcs.go	50.00%	8 Missing ⚠️
engine/tester/fixtures/tx_fixtures.go	33.33%	6 Missing ⚠️
...ions/testabilities/assert_spvwallet_application.go	0.00%	3 Missing ⚠️
...gine/testabilities/fixture_block_header_service.go	57.14%	2 Missing and 1 partial ⚠️
engine/tester/fixtures/users_fixtures.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #839      +/-   ##
==========================================
+ Coverage   46.19%   46.25%   +0.05%     
==========================================
  Files         371      373       +2     
  Lines       17881    17928      +47     
==========================================
+ Hits         8261     8293      +32     
- Misses       8999     9013      +14     
- Partials      621      622       +1     

Flag	Coverage Δ
unittests	46.25% <38.59%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
actions/paymailserver/register.go	100.00% <100.00%> (ø)
...ons/testabilities/fixture_spvwallet_application.go	88.88% <ø> (ø)
engine/testabilities/fixture_configopts.go	100.00% <100.00%> (ø)
server/server.go	7.04% <100.00%> (-1.30%)	⬇️
engine/tester/fixtures/users_fixtures.go	26.56% <0.00%> (-0.43%)	⬇️
...ions/testabilities/assert_spvwallet_application.go	63.76% <0.00%> (-2.90%)	⬇️
...gine/testabilities/fixture_block_header_service.go	69.51% <57.14%> (-1.16%)	⬇️
engine/tester/fixtures/tx_fixtures.go	86.11% <33.33%> (-3.82%)	⬇️
engine/tester/jsonrequire/funcs.go	56.00% <50.00%> (-20.93%)	⬇️
engine/tester/jsonrequire/getter.go	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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