[MRG + 2] EHN additional test for trees regarding fitting behaviour w…

…ith constant features (scikit-learn#8580)

* TST add test checking the behaviour of constant/no-constant features

* FIX/TST factorize test

* TST Add additional constant features

* FIX/TST remove ExtraTree from test

sklearn/tree/tests/test_tree.py

@@ -1186,6 +1186,19 @@ def test_only_constant_features():
         assert_equal(est.tree_.max_depth, 0)
 
 
+def test_behaviour_constant_feature_after_splits():
+    X = np.transpose(np.vstack(([[0, 0, 0, 0, 0, 1, 2, 4, 5, 6, 7]],
+                               np.zeros((4, 11)))))
+    y = [0, 0, 0, 1, 1, 2, 2, 2, 3, 3, 3]
+    for name, TreeEstimator in ALL_TREES.items():
+        # do not check extra random trees
+        if "ExtraTree" not in name:
+            est = TreeEstimator(random_state=0, max_features=1)
+            est.fit(X, y)
+            assert_equal(est.tree_.max_depth, 2)
+            assert_equal(est.tree_.node_count, 5)
+
+
 def test_with_only_one_non_constant_features():
     X = np.hstack([np.array([[1.], [1.], [0.], [0.]]),
                    np.zeros((4, 1000))])