Add a new test checking wxGraphicsContext implementation correctness.

This test verifies that the output of wxGraphicsContext on the current system
matches the pregenerated reference output.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76164 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775