test(yaml): test handling of omap

[kt3k]

This PR adds test cases of yaml, which exercises the handling omap type in yaml string.

(yaml/_type/omap.ts is covered 100% with this change)

related: #3713

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.35%. Comparing base (5d7cb3c) to head (0628b09).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4851      +/-   ##
==========================================
+ Coverage   92.26%   92.35%   +0.09%     
==========================================
  Files         487      487              
  Lines       41535    41531       -4     
  Branches     5394     5405      +11     
==========================================
+ Hits        38324    38358      +34     
+ Misses       3154     3117      -37     
+ Partials       57       56       -1     

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