test(yaml): test block scalar output of stringify

[kt3k]

This PR adds test cases which exercise block scalar output of stringify

part of #3713

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.93%. Comparing base (a578761) to head (07ac0ff).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5341      +/-   ##
==========================================
+ Coverage   95.80%   95.93%   +0.13%     
==========================================
  Files         458      458              
  Lines       37871    37861      -10     
  Branches     5562     5579      +17     
==========================================
+ Hits        36283    36323      +40     
+ Misses       1548     1497      -51     
- Partials       40       41       +1     

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