added backwards compatibility tests

Author: wolph

tests/test_backwards_compatibility.py

@@ -0,0 +1,16 @@
+import time
+import progressbar
+
+
+def test_progressbar_1_widgets():
+    widgets = [
+        progressbar.AdaptiveETA(format="Time left: %s"),
+        progressbar.Timer(format="Time passed: %s"),
+        progressbar.Bar()
+    ]
+
+    bar = progressbar.ProgressBar(widgets=widgets, max_value=100).start()
+
+    for i in range(1, 101):
+        bar.update(i)
+        time.sleep(0.1)