Add multi-output test.

Author: T4rk1n

tests/test_integration.py

@@ -561,12 +561,16 @@ def on_click(n_clicks, n_clicks_timestamp):
 
             return n_clicks, n_clicks_timestamp
 
+        self.startServer(app)
 
         t = time.time()
 
         btn = self.wait_for_element_by_id('output-btn')
         btn.click()
         time.sleep(1)
 
-        self.wait_for_text_to_equals()
+        self.wait_for_text_to_equal('#output1', '1')
+        output2 = self.wait_for_element_by_css_selector('#output2')
+
+        self.assertGreater(int(output2.text), t)