Correct a mockery path that was preventing forcing the darwin platform in windowsTest

[petemill]

This was causing unittests to fail on linux and windows

Fix #12090

Issue was introduced with #11764 which was merged to master, 0.21.x and 0.20.x

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:
unit tests pass

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[codecov-io]

Codecov Report

Merging #12091 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #12091      +/-   ##
==========================================
+ Coverage   54.53%   54.56%   +0.03%     
==========================================
  Files         275      275              
  Lines       26153    26153              
  Branches     4196     4196              
==========================================
+ Hits        14263    14271       +8     
+ Misses      11890    11882       -8

Flag	Coverage Δ
#unittest	54.56% <ø> (+0.03%)	⬆️

Impacted Files	Coverage Δ
app/browser/windows.js	37.53% <0%> (+2.09%)	⬆️

[NejcZdovc]

++, thanks for fixing it

[NejcZdovc]

master 82544bb
0.21 3ff52e3
0.20 166cb82