Move the "empty search for latest" to collections

There is now a "latest" collection (or "latest_nsfw").
The feature didn't look much used, since it didn't work on many
backends.
Using collections will make it easy to support other things
like most viewed, featured, etc.
As a bonus, I added tests for every backend with the feature.

modules/youtube/test.py

@@ -19,11 +19,13 @@
 
 
 from weboob.tools.test import BackendTest
+from weboob.capabilities.video import BaseVideo
+
 
 class YoutubeTest(BackendTest):
     BACKEND = 'youtube'
 
-    def test_youtube(self):
+    def test_search(self):
         l = list(self.backend.search_videos('lol'))
         self.assertTrue(len(l) > 0)
         v = l[0]
@@ -31,3 +33,7 @@ class YoutubeTest(BackendTest):
         self.assertTrue(v.url and v.url.startswith('http://'), 'URL for video "%s" not found: %s' % (v.id, v.url))
         assert self.backend.get_video(v.shorturl)
         self.backend.browser.openurl(v.url)
+
+    def test_latest(self):
+        l = list(self.backend.iter_resources([BaseVideo], [u'latest']))
+        assert len(l) > 0