Add tests for youtube channel urls

They have to be in the form "https://www.youtube.com/channel/ID"

Author: Stypox

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

@@ -111,7 +111,7 @@ public void testGetUploadDate() throws ParsingException {
 
         @Test
         public void testGetUploaderUrl() throws ParsingException {
-            assertTrue(extractor.getUploaderUrl().length() > 0);
+            assertEquals("https://www.youtube.com/channel/UCsRM0YB_dabtEPGPTKo-gcw", extractor.getUploaderUrl());
         }
 
         @Test

extractor/src/test/java/org/schabi/newpipe/extractor/services/youtube/search/YoutubeSearchExtractorChannelOnlyTest.java

@@ -1,5 +1,6 @@
 package org.schabi.newpipe.extractor.services.youtube.search;
 
+import org.hamcrest.CoreMatchers;
 import org.junit.BeforeClass;
 import org.junit.Ignore;
 import org.junit.Test;
@@ -63,4 +64,16 @@ public void testOnlyContainChannels() {
             }
         }
     }
+
+    @Test
+    public void testChannelUrl() {
+        for(InfoItem item : itemsPage.getItems()) {
+            if (item.getName().contains("PewDiePie")) {
+                assertEquals("https://www.youtube.com/channel/UC-lHJZR3Gqxm24_Vd_AJ5Yw", item.getUrl());
+                break;
+            } else {
+                assertThat(item.getUrl(), CoreMatchers.startsWith("https://www.youtube.com/channel/"));
+            }
+        }
+    }
 }