jme3-plugins imports NormalTangentTest incorrectly

[stephengold]

I noticed a defect in GltfLoader while developing the MonkeyWrench library.

The test data is NormalTangentTest from Khronos's glTF sample models.

Here's how it looks in CompareLoaders when "NormalTangentTest/glTF/NormalTangentTest.gltf" is loaded using jme3-plugins:

According to the documentation for the sample model, this result indicates that the sign of the Y component of the normal is reversed.

I haven't identified the root cause of this bug. However, since MonkeyWrench passes this same test, it's likely the bug is in jme3-plugins, not the PBR material.