test: improve coverage on collapsible tree node sibling test

[davidlj95]

#891 failed due to low patch coverage. However the issue is actually that CollapsibleTreeNode.collapseAllChildren isn't covered. There's a test about it, but doesn't use the actual implementation due to the use of a spy.

To actually test the method works as expected, refactoring the test so that method implementation isn't spied. Moves the test apart as it requires a bit of special setup.

[davidlj95]

• test: improve coverage on collapsible tree node sibling test #893 👈 (View in Graphite)
• main

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.72%. Comparing base (1389835) to head (7b948b0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #893      +/-   ##
==========================================
+ Coverage   96.23%   96.72%   +0.49%     
==========================================
  Files         124      124              
  Lines         611      611              
  Branches       68       68              
==========================================
+ Hits          588      591       +3     
+ Misses         19       16       -3     
  Partials        4        4              

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