[youtube] Re-add age protection test (#14)

Co-authored-by: Dominika <sokolov.dominika@gmail.com>

youtube_dl/extractor/youtube.py

@@ -578,6 +578,22 @@ class YoutubeIE(YoutubeBaseInfoExtractor):
                 'end_time': 9,
             }
         },
+        {
+            'url': 'https://www.youtube.com/watch?v=PA4gbtKWNAI',
+            'note': 'Test video with age protection',
+            'info_dict': {
+                'id': 'PA4gbtKWNAI',
+                'ext': 'mp4',
+                'upload_date': '20201028',
+                'title': 'Big Buck Bunny (TEST) (Age-restricted)',
+                'description': 'An age restricted test video for youtube-dl',
+                'duration': 635,
+                'uploader': 'Ali Sherief',
+                'uploader_id': 'UCi1TsEkfcMaYSadGms3UnqA',
+                'uploader_url': r're:https?://(?:www\.)?youtube\.com/channel/UCi1TsEkfcMaYSadGms3UnqA',
+                'age_limit': 18,
+            }
+        },
         {
             'url': '//www.YouTube.com/watch?v=yZIXLfi8CZQ',
             'note': 'Embed-only video (#1746)',