chore: add backwards multiline search test

[akoreman]

Issue #, if available: NA

Description of changes: Follow up to #5675

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Pull Request Checklist:

• [na] No backwards incompatible changes were made to Ace's public interface which is defined through the main typings file (ace.d.ts) and its references:
  • https://github.com/ajaxorg/ace/blob/master/ace.d.ts
  • https://github.com/ajaxorg/ace/blob/master/ace-modes.d.ts
  • https://github.com/ajaxorg/ace/blob/master/ace-extensions.d.ts

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.10%. Comparing base (06d51b9) to head (b737d8c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5751      +/-   ##
==========================================
+ Coverage   87.02%   87.10%   +0.07%     
==========================================
  Files         602      602              
  Lines       44087    44094       +7     
  Branches     7233     7233              
==========================================
+ Hits        38368    38408      +40     
+ Misses       5719     5686      -33     

Flag	Coverage Δ
unittests	87.10% <100.00%> (+0.07%)	⬆️

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