[YouTube] Enable and change testRelatedItems method of YoutubePlaylistExtractorTest.LearningPlaylist

This change makes the method test whether there is no more items instead of
being disabled.

extractor/src/test/java/org/schabi/newpipe/extractor/services/youtube/YoutubePlaylistExtractorTest.java

@@ -342,10 +342,9 @@ public class YoutubePlaylistExtractorTest {
             defaultTestRelatedItems(extractor);
         }
 
-        @Disabled
         @Test
         public void testMoreRelatedItems() throws Exception {
-            defaultTestMoreItems(extractor);
+            assertFalse(extractor.getInitialPage().hasNextPage());
         }
 
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