Fix -Wunused-but-set-variable in transfer function tests

Since the tests only exercise the method to make sure there is no
crash, adding "Q_UNUSED" is good enough.

Fixes #209
Merge branch '209-fix-vtk-transfer-function-test-warnings'

* 209-fix-vtk-transfer-function-test-warnings:
  Fix -Wunused-but-set-variable in transfer function tests