[Bandcamp] Add utility test method to test images

This method, testImages(Collection<Image>), will use first the default image
collection test in DefaultTests and then will check that each image URL
contains f4.bcbits.com/img and ends with .jpg or .png.

To do so, a new non-instantiable final class has been added: BandcampTestUtils.

extractor/src/test/java/org/schabi/newpipe/extractor/services/bandcamp/BandcampTestUtils.java

@@ -0,0 +1,34 @@
+package org.schabi.newpipe.extractor.services.bandcamp;
+
+import org.schabi.newpipe.extractor.Image;
+import org.schabi.newpipe.extractor.services.DefaultTests;
+
+import javax.annotation.Nullable;
+import java.util.Collection;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * Utility class for Bandcamp tests.
+ */
+public final class BandcampTestUtils {
+    private BandcampTestUtils() {
+    }
+
+    /**
+     * Test that Bandcamp images of a {@link Collection} respect
+     * {@link DefaultTests#defaultTestImageCollection(Collection) default requirements}, contain
+     * the string {@code f4.bcbits.com/img} in their URL and end with {@code .jpg} or {@code .png}.
+     *
+     * @param images a Bandcamp {@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
+        assertTrue(images.stream()
+                .allMatch(image -> image.getUrl().contains("f4.bcbits.com/img")
+                        && (image.getUrl().endsWith(".jpg") || image.getUrl().endsWith(".png"))));
+    }
+}