Fix Python solver error message regression test

Update the regression assertion to match the current actionable\nbinding message raised when set_initial_trajectory is called\nbefore a dynamical system is configured.

Author: astomodynamics

python/tests/test_solver_errors.py

@@ -61,7 +61,7 @@ def test_set_initial_trajectory_requires_dynamical_system():
     X = [np.zeros(2) for _ in range(solver.horizon + 1)]
     U = [np.zeros(1) for _ in range(solver.horizon)]
 
-    with pytest.raises(ValueError, match="requires a dynamical system"):
+    with pytest.raises(ValueError, match="is a dynamical system set"):
         solver.set_initial_trajectory(X, U)