chore(descriptor): justify ignored tests and add missing assertions

[luisschwab]

Description

Closes #95.

This PR adds reasoning to the ignored policy extraction tests (see bdk#225 for context), and adds missing assertions on a couple of timelock merging tests that were missing.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before pushing

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.30%. Comparing base (8f8a8e9) to head (3cc5b98).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #369      +/-   ##
==========================================
+ Coverage   85.33%   86.30%   +0.96%     
==========================================
  Files          24       24              
  Lines        8335     8346      +11     
==========================================
+ Hits         7113     7203      +90     
+ Misses       1222     1143      -79     

Flag	Coverage Δ
rust	86.30% <100.00%> (+0.96%)	⬆️

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

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.