Add tests for the on_demand option

IPython/html/widgets/tests/test_interaction.py

@@ -480,3 +480,16 b' def test_custom_description():'
         value='text',
         description='foo',
     )
+
+def test_interact_ondemand_button():
+    c = interactive(f, on_demand=True)
+    w = c.children[0]
+    check_widget(w, cls=widgets.ButtonWidget)
+
+def test_interact_ondemand_nocall():
+    callcount = 0
+    def calltest(testarg):
+        callcount += 1
+    c = interactive(calltest, testarg=5, on_demand=True)
+    c.children[0].value = 10
+    nt.assert_equal(callcount, 0)