add t0<tf constraint in test case with both free times

test/test_objective.jl

@@ -58,7 +58,7 @@ constraint!(ocp4, :initial, [0,0], :initial_constraint)
 constraint!(ocp4, :final, [1,0], :final_constraint)
 constraint!(ocp4, :control, -1, 1, :control_constraint)
 constraint!(ocp4, :variable, [0.1, 0.1], [10, 10], :variable_constraint)
-constraint!(ocp4, :variable, v -> v[2]-v[1], 0, Inf )
+constraint!(ocp4, :variable, v -> v[2]-v[1], 1.0, Inf )
 dynamics!(ocp4, (x, u, v) ->  [x[2], u])
 objective!(ocp4, :mayer, (x0, xf, v) -> v[1], :max)
 sol4 = solve(ocp4, grid_size=100, print_level=0, tol=1e-12)