Add reason field to MockOnly

This enforces developers to document why a test is skipped

extractor/src/test/java/org/schabi/newpipe/MockOnly.java

@@ -6,6 +6,8 @@ import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
 
+import javax.annotation.Nonnull;
+
 /**
  * Marker annotation to skip test if it not run with mocks.
  *
@@ -15,4 +17,9 @@ import java.lang.annotation.Target;
 @Target({ElementType.METHOD, ElementType.TYPE})
 @Inherited
 public @interface MockOnly {
+
+    /**
+     * Explanation why this test shold only be run with mocks and not against real websites
+     */
+    @Nonnull String reason();
 }

extractor/src/test/java/org/schabi/newpipe/MockOnlyRule.java

@@ -33,13 +33,15 @@ public class MockOnlyRule implements TestRule {
         return new Statement() {
             @Override
             public void evaluate() throws Throwable {
-                final boolean hasAnnotation = description.getAnnotation(MockOnly.class) == null;
-                final boolean isMockDownloader = downloader == null ||
-                        !downloader.equalsIgnoreCase(DownloaderType.REAL.toString());
-                Assume.assumeTrue(
-                        "The test is not reliable against a website and thus skipped",
-                        hasAnnotation && isMockDownloader
-                );
+                final MockOnly annotation = description.getAnnotation(MockOnly.class);
+                if (annotation != null) {
+                    final boolean isMockDownloader = downloader == null ||
+                            !downloader.equalsIgnoreCase(DownloaderType.REAL.toString());
+
+                    Assume.assumeTrue("The test is not reliable against real website. Reason: "
+                            + annotation.reason(), isMockDownloader);
+                }
+
 
                 base.evaluate();
             }