We have some new volume rendering tests depend on runtime state, and
we cannot know whether or not the required support will be there
during configure time.

This patch adds a new symbolic constant, VTK_SKIP_RETURN_CODE that
uses the SKIP_RETURN_CODE cmake test property, which allows us to skip
the test at runtime when invalid configurations are detected. The
value of 125 was chosen to match git rebase's skip value.