Fix: replace t_end by t_start to reverse operator in utils.by

Author: speglich

tests/utils.py

@@ -128,6 +128,6 @@ def apply(self, **kwargs):
                 past_idx = (t-1) % np.shape(self.u)[0]
                 self.u[idx][:] = self.u[past_idx][:] + self.direction * 1
         else:
-            idx = t_end % np.shape(self.u)[0]
+            idx = (t_start) % np.shape(self.u)[0]
             self.v[:] = (self.u[idx][:]*(-1) + 1)
         self.counter += abs(t_end - t_start)