fix test exception

com.unity.ml-agents/Tests/Editor/InplaceArrayTests.cs

                 case 4:
                     return new InplaceArray<int>(11, 22, 33, 44);
                 default:
-                    throw new RuntimeException("bad test!");
+                    throw new ArgumentException("bad test!");
             }
         }
 
                 case 4:
                     return new InplaceArray<int>(0, 0, 0, 0);
                 default:
-                    throw new RuntimeException("bad test!");
+                    throw new ArgumentException("bad test!");
             }
         }