[YouTube] Add utility test method to test images in YoutubeTestsUtils

This method, testImages(Collection<Image>), will use first the default image
collection test in DefaultTests and then will check that each image URL
contains the string yt.

The JavaDoc of the class has been also updated to reflect the changes made in
it (it is now more general).

Author: AudricV

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

@@ -1,11 +1,16 @@
 package org.schabi.newpipe.extractor.services.youtube;
 
+import org.schabi.newpipe.extractor.ExtractorAsserts;
+import org.schabi.newpipe.extractor.Image;
+import org.schabi.newpipe.extractor.services.DefaultTests;
 import org.schabi.newpipe.extractor.services.youtube.extractors.YoutubeStreamExtractor;
 
+import javax.annotation.Nullable;
+import java.util.Collection;
 import java.util.Random;
 
 /**
- * Utility class for keeping YouTube tests stateless.
+ * Utility class for YouTube tests.
  */
 public final class YoutubeTestsUtils {
     private YoutubeTestsUtils() {
@@ -26,4 +31,20 @@ public static void ensureStateless() {
         YoutubeParsingHelper.setNumberGenerator(new Random(1));
         YoutubeStreamExtractor.resetDeobfuscationCode();
     }
+
+    /**
+     * Test that YouTube images of a {@link Collection} respect
+     * {@link DefaultTests#defaultTestImageCollection(Collection) default requirements} and contain
+     * the string {@code yt} in their URL.
+     *
+     * @param images a YouTube {@link Image} {@link Collection}
+     */
+    public static void testImages(@Nullable final Collection<Image> images) {
+        DefaultTests.defaultTestImageCollection(images);
+        // Disable NPE warning because if the collection is null, an AssertionError would be thrown
+        // by DefaultTests.defaultTestImageCollection
+        //noinspection DataFlowIssue
+        images.forEach(image ->
+            ExtractorAsserts.assertContains("yt", image.getUrl()));
+    }
 }