output/recorder, test/*: invoke encoder_read() after _open()

Make sure the file header gets written at the beginning, before
_write() gets called.

src/encoder_plugin.h

@@ -119,6 +119,10 @@ encoder_finish(struct encoder *encoder)
  * Before you free it, you must call encoder_close().  You may open
  * and close (reuse) one encoder any number of times.
  *
+ * After this function returns successfully and before the first
+ * encoder_write() call, you should invoke encoder_read() to obtain
+ * the file header.
+ *
  * @param encoder the encoder
  * @param audio_format the encoder's input audio format; the plugin
  * may modify the struct to adapt it to its abilities

src/output/recorder_output_plugin.c

@@ -192,6 +192,13 @@ recorder_output_open(struct audio_output *ao,
 		return false;
 	}
 
+	if (!recorder_output_encoder_to_file(recorder, error_r)) {
+		encoder_close(recorder->encoder);
+		close(recorder->fd);
+		unlink(recorder->path);
+		return false;
+	}
+
 	return true;
 }