Fix ModelBuilderVerifyAttributeSavePy test
Various updates prompted by this test
- Update TEST_ENV to current paraview version (was hard-coded to paraview-5.0)
- Strip env vars when ModelBuilder is run from python python subprocess call - no longer needed with today's superbuild, and screws things up in some cases.
- Update path to test2D.cmb file to current cmb-testing-data
- Fix mat2 association (presumably because model-labeling changed)