Update test_semilinear1d.py

Author: milzj

tests/algorithms/test_semilinear1d.py

@@ -233,7 +233,8 @@ def test_convergence_rate():
 
     # Convergence dual gap
     rates = convergence_rates(dual_gaps, [1.0/n for n in ns])
-
+    assert dual_gaps < 0
+    
     assert np.isclose(np.median(rates), 2.0, atol=0.2)
 
     X = np.ones((np.size(ns), 2)); X[:, 1] = np.log([1.0/n for n in ns])