Tests: Add BufferAttribute.toJSON() unit test

[takahirox]

Description

This PR adds BufferAttribute.toJSON() unit test.

I realized that .name and .usage are not serialized. Is it intentional because .name is optional and (if I'm right) isn't used in core, and appropriate .usage should depend on app, not data?

This contribution is made in a hospital.

[Mugen87]

Since name is actually deserialized in BufferGeometryLoader, I'm a bit surprised that it is missing in toJSON().

three.js/src/loaders/BufferGeometryLoader.js

Line 125 in 6a4af45

if ( attribute.name !== undefined ) bufferAttribute.name = attribute.name;

I guess it would be more consistent to honor name, usage and updateRange in toJSON(). And correctly restore them.

[mrdoob]

Thanks!

[takahirox]

I guess it would be more consistent to honor name, usage and updateRange in toJSON(). And correctly restore them.

OK, I'll work on it. I didn't look into deserialization side.