Add `"WebGL"` to unit tests

[j9liu]

This PR adds the missing "WebGL" keyword to various ModelExperimental specs for consistency with the rest of Cesium's unit tests.

[cesium-concierge]

Thanks for the pull request @j9liu!

• :heavy_check_mark: Signed CLA found.

Reviewers, don't forget to make sure that:

• [ ] Cesium Viewer works.
• [ ] Works in 2D/CV.

[ptrgags]

@j9liu could you update this for the latest main? Then I can review this today.

[j9liu]

@ptrgags updated, I just redid my changes on top of the latest update on main. I left ModelDrawCommand and ClassificationModelDrawCommand untouched since those will be affected by #10716.

In addition to adding "WebGL" to all the specs, I also changed the variable names in I3dmLoaderSpec to be camelCase in accordance with the Coding Guide.

[ptrgags]

Looks good, thanks @j9liu!