Update BaseExtractorTests image methods' name

Also suppress unused warnings in BaseStreamExtractorTest, like it is done on
other BaseExtractorTests interfaces.

Author: AudricV

extractor/src/test/java/org/schabi/newpipe/extractor/services/BaseChannelExtractorTest.java

@@ -6,9 +6,9 @@ public interface BaseChannelExtractorTest extends BaseExtractorTest {
     @Test
     void testDescription() throws Exception;
     @Test
-    void testAvatarUrl() throws Exception;
+    void testAvatars() throws Exception;
     @Test
-    void testBannerUrl() throws Exception;
+    void testBanners() throws Exception;
     @Test
     void testFeedUrl() throws Exception;
     @Test

extractor/src/test/java/org/schabi/newpipe/extractor/services/BasePlaylistExtractorTest.java

@@ -4,13 +4,13 @@
 
 public interface BasePlaylistExtractorTest extends BaseListExtractorTest {
     @Test
-    void testThumbnailUrl() throws Exception;
+    void testThumbnails() throws Exception;
     @Test
-    void testBannerUrl() throws Exception;
+    void testBanners() throws Exception;
     @Test
     void testUploaderName() throws Exception;
     @Test
-    void testUploaderAvatarUrl() throws Exception;
+    void testUploaderAvatars() throws Exception;
     @Test
     void testStreamCount() throws Exception;
     @Test

extractor/src/test/java/org/schabi/newpipe/extractor/services/BaseStreamExtractorTest.java

@@ -2,6 +2,7 @@
 
 import org.junit.jupiter.api.Test;
 
+@SuppressWarnings("unused")
 public interface BaseStreamExtractorTest extends BaseExtractorTest {
     @Test
     void testStreamType() throws Exception;
@@ -10,17 +11,17 @@ public interface BaseStreamExtractorTest extends BaseExtractorTest {
     @Test
     void testUploaderUrl() throws Exception;
     @Test
-    void testUploaderAvatarUrl() throws Exception;
+    void testUploaderAvatars() throws Exception;
     @Test
     void testSubscriberCount() throws Exception;
     @Test
     void testSubChannelName() throws Exception;
     @Test
     void testSubChannelUrl() throws Exception;
     @Test
-    void testSubChannelAvatarUrl() throws Exception;
+    void testSubChannelAvatars() throws Exception;
     @Test
-    void testThumbnailUrl() throws Exception;
+    void testThumbnails() throws Exception;
     @Test
     void testDescription() throws Exception;
     @Test