Use GuiTestAssistant in TestSplitEditorAreaPane

[corranwebster]

Fixes #463 - the test was not using GuiTestAssistant and so was not cleaning up after itself.

[codecov-io]

Codecov Report

Merging #464 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #464   +/-   ##
=======================================
  Coverage   36.54%   36.54%           
=======================================
  Files         481      481           
  Lines       26422    26422           
  Branches     3969     3969           
=======================================
  Hits         9655     9655           
  Misses      16334    16334           
  Partials      433      433

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[corranwebster]

Closing in favour of #1203