Add more tests for skin animation

[stevk]

• Joint hierarchy has no common root. ([Clarification] Skeleton root and joints hierarchy glTF#1270)
  As discussed, the skeleton property should probably be removed, and can already be ignored by implementations. In addition to that, we should have an example asset for which no common root is possible (e.g. bones form two subtrees starting from the scene root).
  not skeleton property is kept.

• Multiple meshes sharing a single skin. (Sharing skeletons between two skinned mesh glTF#1285)

• Many (>4) joint vertex influences. (Skinning influenced by more than 4 joints glTF#1381)