ENH: Add all GPURayCast tests for OpenGL2 backend

If OpenGL2 backend is selected, only and all the GPURayCast tests are enabled.
If VolumeOpenGLNew is selected, all GPURayCast along with other VolumeOpenGL
tests are enabled.

Change-Id: Ia571fe10a1c0ac05df6433278190e7f108768a7c