Add materials test to ABConverter visual test coverage

In this issue, we had a false positive because the affected geometry is inside a big mesh:
https://github.com/decentraland/unity-renderer/issues/630

To solve this issue, the safest way is to visual test each asset bundle material on a primitive, like a sphere and/or cube. We can compare the materials between asset bundles and gltfs using this method to get a more reliable test.