Fix OP_RUN test case

My did my numbers wrong. 0b1111 is definitely not 13. Not under normal
encodings, not with the -1 OP_RUN bias. Bro what.

src/main.rs

@@ -368,7 +368,7 @@ mod test {
     fn decoder_unpack_run() {
         let compressed = [
             (QOI_OP_RUN | 0b0000_0000), //  1 -- bias of -1, so all zeros is a run of 1 pixel
-            (QOI_OP_RUN | 0b0000_1111), // 13
+            (QOI_OP_RUN | 0b0000_1100), // 13 // 0b1111?  no it isn't. what?
         ];
 
         let init_pixel = PixelRGBA::new(50, 100, 150, 200);