fix: Added mockdom.before method

[andrewnester]

Issue #, if available:

Description of changes:
Brought back from c9

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[codecov]

Codecov Report

Merging #4724 (7e298ed) into master (bf2c7e2) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4724      +/-   ##
==========================================
+ Coverage   71.19%   71.24%   +0.04%     
==========================================
  Files         553      553              
  Lines       55598    55598              
  Branches    10392    10392              
==========================================
+ Hits        39584    39611      +27     
+ Misses      16014    15987      -27     

Flag	Coverage Δ
unittests	71.24% <ø> (+0.04%)	⬆️

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

Impacted Files	Coverage Δ
lib/ace/range.js	84.82% <0.00%> (-0.69%)	⬇️
lib/ace/editor.js	81.61% <0.00%> (+1.97%)	⬆️

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[andredcoliveira]

Looks good to me