Add MockOnly junit 5 test extension

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

@@ -0,0 +1,20 @@
+package org.schabi.newpipe.downloader;
+
+import org.junit.jupiter.api.extension.ExtendWith;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+
+/**
+ * Use this to annotate tests methods/classes that should only be run when the downloader is of type
+ * {@link DownloaderType#MOCK} or {@link DownloaderType#RECORDING}. This should be used when e.g. an
+ * extractor returns different results each time because the underlying service web page does so. In
+ * that case it makes sense to only run the tests with the mock downloader, since the real web page
+ * is not reliable, but we still want to make sure that the code correctly interprets the stored and
+ * mocked web page data.
+ * @see MockOnlyCondition
+ */
+@Retention(RetentionPolicy.RUNTIME)
+@ExtendWith(MockOnlyCondition.class)
+public @interface MockOnly {
+}

extractor/src/test/java/org/schabi/newpipe/downloader/MockOnlyCondition.java

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+package org.schabi.newpipe.downloader;
+
+import org.junit.jupiter.api.extension.ConditionEvaluationResult;
+import org.junit.jupiter.api.extension.ExecutionCondition;
+import org.junit.jupiter.api.extension.ExtensionContext;
+
+/**
+ * @see MockOnly
+ */
+public class MockOnlyCondition implements ExecutionCondition {
+    private static final String MOCK_ONLY_REASON = "Mock only";
+
+    @Override
+    public ConditionEvaluationResult evaluateExecutionCondition(final ExtensionContext context) {
+        if (DownloaderFactory.getDownloaderType() == DownloaderType.REAL) {
+            return ConditionEvaluationResult.disabled(MOCK_ONLY_REASON);
+        } else {
+            return ConditionEvaluationResult.enabled(MOCK_ONLY_REASON);
+        }
+    }
+}