refactor(RestructuredTest): rename variable, fix typo and remove unnecessary string copy

[bearomorphism]

Description

• Rename variables
• Fix the unbounded variable index
• Make some methods module functions
• Fix typo
• Remove the use of string slice (e.g. first[1:]) because it causes extra string copy

Similar changes in #1596

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@            Coverage Diff            @@
##             v4-9-2    #1609   +/-   ##
=========================================
  Coverage          ?   98.25%           
=========================================
  Files             ?       58           
  Lines             ?     2690           
  Branches          ?        0           
=========================================
  Hits              ?     2643           
  Misses            ?       47           
  Partials          ?        0           

Flag	Coverage Δ
unittests	98.25% <100.00%> (?)

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