Add MockOnlyRule to allow skipping specific tests based on downloader

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

@@ -0,0 +1,18 @@
+package org.schabi.newpipe;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Inherited;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Marker annotation to skip test if it not run with mocks.
+ *
+ * {@link MockOnlyRule}
+ */
+@Retention(RetentionPolicy.RUNTIME)
+@Target({ElementType.METHOD, ElementType.TYPE})
+@Inherited
+public @interface MockOnly {
+}

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

@@ -0,0 +1,48 @@
+package org.schabi.newpipe;
+
+import org.junit.Assume;
+import org.junit.rules.TestRule;
+import org.junit.runner.Description;
+import org.junit.runners.model.Statement;
+import org.schabi.newpipe.downloader.DownloaderType;
+
+import javax.annotation.Nonnull;
+
+/**
+ * <p>
+ * Checks if the system variable "downloader" is set to "REAL" and skips the tests if it is.
+ * Otherwise execute the test.
+ * </p>
+ *
+ * <p>
+ * Use it by creating a public variable of this inside the test class and annotate it with
+ * {@link org.junit.Rule}. Then annotate the tests to be skipped with {@link MockOnly}
+ * </p>
+ *
+ * <p>
+ * Allows skipping unreliable or time sensitive tests in CI pipeline.
+ * </p>
+ */
+public class MockOnlyRule implements TestRule {
+
+    final String downloader = System.getProperty("downloader");
+
+    @Override
+    @Nonnull
+    public Statement apply(@Nonnull Statement base, @Nonnull Description description) {
+        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
+                );
+
+                base.evaluate();
+            }
+        };
+    }
+}