[test] unittest 修正

[ousttrue]

Unlit/Color だと例外が発生する。
2022.3 に更新して挙動が変わったかもしれない(未確認)。

Material `Unlit/Color` fallbacks.
Material 'Unlit/Color' with Shader 'Unlit/Color' doesn't have a texture property '_MainTex'
System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.

var index = textureExporter.RegisterExportingAsSRgb(src.mainTexture, false); // GetTexture("_MainTex");

UniVRM/Assets/UniGLTF/Runtime/UniGLTF/IO/MaterialIO/URP/Export/Materials/UrpFallbackMaterialExporter.cs

Line 22 in d3439ab

var index = textureExporter.RegisterExportingAsSRgb(src.mainTexture, false);