Fixed Vector.cross and optimized the Vector tests slightly.

src/main/java/com/sk89q/worldedit/Vector.java

@@ -503,9 +503,9 @@ public class Vector {
      */
     public Vector cross(Vector other) {
         return new Vector(
-            y * other.z + z * other.y,
-            z * other.x + x * other.z,
-            x * other.y + y * other.x
+            y * other.z - z * other.y,
+            z * other.x - x * other.z,
+            x * other.y - y * other.x
         );
     }