Explain how to overwrite golden data in extractor test failure message

This info is already in the javadoc of FakeExtractorOutput#assertOutput
but it's not super discoverable if you just see the test failure. I was
manually copying the dump results around before realising there was
a mechanism to auto-update.

PiperOrigin-RevId: 296204463

testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeExtractorOutput.java

@@ -129,7 +129,14 @@ public final class FakeExtractorOutput implements ExtractorOutput, Dumper.Dumpab
 
     if (DUMP_FILE_ACTION == COMPARE_WITH_EXISTING) {
       String expected = TestUtil.getString(context, dumpFile);
-      assertWithMessage(dumpFile).that(actual).isEqualTo(expected);
+      assertWithMessage(
+              "Extractor output doesn't match golden file: %s\n"
+                  + "To update the golden, change FakeExtractorOutput#DUMP_FILE_ACTION to"
+                  + " WRITE_TO_LOCAL (for Robolectric tests) or WRITE_TO_DEVICE (for"
+                  + " instrumentation tests) and re-run the test.",
+              dumpFile)
+          .that(actual)
+          .isEqualTo(expected);
     } else {
       File file =
           DUMP_FILE_ACTION == WRITE_TO_LOCAL