ci: add silent MP3 file and test for decoder silence (#411)

Author: roderickvd

assets/silence.mp3

@@ -0,0 +1,9 @@
+#[cfg(any(feature = "minimp3", feature = "symphonia-mp3"))]
+#[test]
+fn test_silent_mp3() {
+    let file = std::fs::File::open("assets/silence.mp3").unwrap();
+    let mut decoder = rodio::Decoder::try_from(file).unwrap();
+
+    // File is just silence
+    assert!(decoder.all(|x| x < 0.0001));
+}