[youtube:user] Require 'https?://' in the url (fixes #8356)

It was matching www.youtube.com/embed/WpfukLMe1TM.
The generic extractor automatically adds http:// if it's missing.

test/test_all_urls.py

@@ -56,7 +56,7 @@ class TestAllURLsMatching(unittest.TestCase):
         assertChannel('https://www.youtube.com/channel/HCtnHdj3df7iM/videos')
 
     def test_youtube_user_matching(self):
-        self.assertMatch('www.youtube.com/NASAgovVideo/videos', ['youtube:user'])
+        self.assertMatch('http://www.youtube.com/NASAgovVideo/videos', ['youtube:user'])
 
     def test_youtube_feeds(self):
         self.assertMatch('https://www.youtube.com/feed/watch_later', ['youtube:watchlater'])

youtube_dl/extractor/youtube.py

@@ -1846,7 +1846,7 @@ class YoutubeChannelIE(YoutubePlaylistBaseInfoExtractor):
 
 class YoutubeUserIE(YoutubeChannelIE):
     IE_DESC = 'YouTube.com user videos (URL or "ytuser" keyword)'
-    _VALID_URL = r'(?:(?:(?:https?://)?(?:\w+\.)?youtube\.com/(?:user/)?(?!(?:attribution_link|watch|results)(?:$|[^a-z_A-Z0-9-])))|ytuser:)(?!feed/)(?P<id>[A-Za-z0-9_-]+)'
+    _VALID_URL = r'(?:(?:https?://(?:\w+\.)?youtube\.com/(?:user/)?(?!(?:attribution_link|watch|results)(?:$|[^a-z_A-Z0-9-])))|ytuser:)(?!feed/)(?P<id>[A-Za-z0-9_-]+)'
     _TEMPLATE_URL = 'https://www.youtube.com/user/%s/videos'
     IE_NAME = 'youtube:user'