Fix incorrect test packet

The packet had 0x12 for the length, which is not the same as the
variable name suggests.

Signed-off-by: Thibaut Vandervelden <thvdveld@vub.be>

src/wire/ipv6hopbyhop.rs

@@ -219,7 +219,7 @@ mod test {
 
     // A Hop-by-Hop Option header with a PadN option of option data length 12.
     static REPR_PACKET_PAD12: [u8; 16] = [
-        0x06, 0x1, 0x1, 0x12, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
+        0x06, 0x1, 0x1, 0x0C, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
     ];
 
     #[test]