Tests without WebGL Context

@google/model-viewer v4.0.0 (also tried v3.5.0)

When running tests in CI (GitHub Actions, without a screen), there seems to be an issue i was not having before:
```ts
TypeError: this.threeRenderer is undefined thrown
```
and
```ts
Uncaught TypeError: Cannot read properties of undefined (reading 'xr') thrown
```
which come together with (it seems):
```ts
Chrome Headless 131.0.0.0 (Mac OS 10.15.7) WARN: Error: Error creating WebGL context.
Error: Error creating WebGL context.
at new WebGLRenderer (http://localhost:9876/_karma_webpack_/webpack:/node_modules/three/build/three.module.js:14906:13)
at new Renderer (http://localhost:9876/_karma_webpack_/@google/webpack:/node_modules/@google/model-viewer/lib/three-components/Renderer.js:73:34)
at Function.singleton (http://localhost:9876/_karma_webpack_/@google/webpack:/node_modules/@google/model-viewer/lib/three-components/Renderer.js:103:31)
at new LoadingModelViewerElement (http://localhost:9876/_karma_webpack_/@google/webpack:/node_modules/@google/model-viewer/lib/features/loading.js:177:22)
at new ARModelViewerElement (http://localhost:9876/_karma_webpack_/@google/webpack:/node_modules/@google/model-viewer/lib/features/ar.js:58:13)
at new ControlsModelViewerElement (http://localhost:9876/_karma_webpack_/@google/webpack:/node_modules/@google/model-viewer/lib/features/controls.js:178:13)
at new EnvironmentModelViewerElement (http://localhost:9876/_karma_webpack_/@google/webpack:/node_modules/@google/model-viewer/lib/features/environment.js:37:13)
at new StagingModelViewerElement (http://localhost:9876/_karma_webpack_/@google/webpack:/node_modules/@google/model-viewer/lib/features/staging.js:42:13)
at new SceneGraphModelViewerElement (http://localhost:9876/_karma_webpack_/@google/webpack:/node_modules/@google/model-viewer/lib/features/scene-graph.js:41:13)
at new AnnotationModelViewerElement (http://localhost:9876/_karma_webpack_/@google/webpack:/node_modules/@google/model-viewer/lib/features/annotation.js:35:13)
```