Add concrete unit tests for Skeleton, SkinnedMesh, Sprite, and WebGL3DRenderTarget

[Nandann018-ux]

This PR converts several QUnit.todo placeholders in core 3D objects and render targets into real, passing unit tests, increasing coverage and verifying runtime behavior without changing existing functionality.

Changes included:

WebGL3DRenderTarget

Tests for depth, texture, and isWebGL3DRenderTarget.

Verifies correct 3D texture creation and depth handling.

Sprite

Tests for geometry, material, center, and copy.

Validates that Sprite geometry attributes and anchoring behave as expected.

Ensures copy() correctly duplicates center and preserves material references.

SkinnedMesh

Tests for bindMode, bindMatrix, bindMatrixInverse, and copy.

Confirms that binding modes, matrices, skeleton references, and bounding volumes are correctly copied.

Skeleton

Tests for uuid, bones, boneInverses, boneMatrices, boneTexture, computeBoneTexture, clone, getBoneByName, fromJSON, and toJSON.
Validates that bone arrays, inverses, textures, and serialization behave as expected.

Validation
All new and existing tests pass.
No lint errors or runtime changes.

Why this helps:
Improves test coverage for core library objects.
Verifies important 3D behavior in a way that prevents regressions.
Prepares the repo for safer future changes without changing any runtime behavior.

[Nandann018-ux]

If there's any specific behavior you’d like tested more deeply, feel free to point me to it — I can add more coverage in a follow-up commit.