Refactor tests to use positions instead of hardcoded numbers

Author: alex-mccarthy-unity

com.unity.ml-agents.extensions/Tests/Runtime/Sensors/RigidBodySensorTests.cs

@@ -104,13 +104,13 @@ public void TestBodiesWithJoint()
             {
                 // Model space
                 0f, 0f, 0f, // Root pos
-                13.37f, 0f, 0f, // Middle pos
+                middleGamObj.transform.position.x, 0f, 0f, // Middle pos
                 leafGameObj.transform.position.x, 0f, 0f, // Leaf pos
 
                 // Local space
                 0f, 0f, 0f, // Root pos
-                13.37f, 0f, 0f, // Attached pos
-                4.2f, 0f, 0f, // Leaf pos
+                middleGamObj.transform.localPosition.x, 0f, 0f, // Attached pos
+                leafGameObj.transform.localPosition.x, 0f, 0f, // Leaf pos
 
                 1f, 0f, 0f, // Root vel (relative to virtual root)
                 -1f, 1f, 0f, // Attached vel