Switch test to use GenericConstantMaterial instead of Density ref #2814

This test was broken because it used Density, which came from misc.
Since we don't care about evolving density, I switched it over to use
GenericConstantMaterial, which resolves the dependency issue.
Merge pull request #2832 from bwspenc/test_fix

Switch test to use GenericConstantMaterial instead of Density