added playlist unit-test

tests/test_playlist.py

@@ -0,0 +1,37 @@
+# -*- coding: utf-8 -*-
+import pytest
+from pytube import playlist
+
+
+def test_construct():
+    ob = playlist.Playlist(
+        "https://www.youtube.com/watch?v=m5q2GCsteQs&list="
+        "PL525f8ds9RvsXDl44X6Wwh9t3fCzFNApw")
+    expected = "https://www.youtube.com/" \
+               "playlist?list=" \
+               "PL525f8ds9RvsXDl44X6Wwh9t3fCzFNApw"
+
+    assert ob.construct_playlist_url() == expected
+
+
+def test_link_parse():
+    ob = playlist.Playlist(
+        "https://www.youtube.com/watch?v=m5q2GCsteQs&list="
+        "PL525f8ds9RvsXDl44X6Wwh9t3fCzFNApw")
+
+    expected = ["/watch?v=m5q2GCsteQs",
+                "/watch?v=5YK63cXyJ2Q",
+                "/watch?v=Rzt4rUPFYD4"]
+    assert ob.parse_links() == expected
+
+
+def test_populate():
+    ob = playlist.Playlist(
+        "https://www.youtube.com/watch?v=m5q2GCsteQs&list="
+        "PL525f8ds9RvsXDl44X6Wwh9t3fCzFNApw")
+    expected = ["https://www.youtube.com/watch?v=m5q2GCsteQs",
+                "https://www.youtube.com/watch?v=5YK63cXyJ2Q",
+                "https://www.youtube.com/watch?v=Rzt4rUPFYD4"]
+
+    ob.populate_video_urls()
+    assert ob.video_urls == expected