fixing unit test: frustum depends on THREE.Sphere to be defined

test/unit/unittests_sources.html

@@ -24,9 +24,9 @@
   <script src="../../src/math/Matrix3.js"></script>
   <script src="../../src/math/Matrix4.js"></script>
   <script src="../../src/math/Ray.js"></script>
+  <script src="../../src/math/Sphere.js"></script>
   <script src="../../src/math/Frustum.js"></script>
   <script src="../../src/math/Plane.js"></script>
-  <script src="../../src/math/Sphere.js"></script>
   <script src="../../src/math/Math.js"></script>
   <script src="../../src/math/Triangle.js"></script>