Add a roundtrip test to kill a mutant

There is a mutant found in relative locktime `to_512_second_intervals`.

Add a round trip test to kill the mutant.

units/src/locktime/relative.rs

@@ -307,6 +307,13 @@ mod tests {
         assert_eq!(NumberOf512Seconds::from_512_second_intervals(1).to_seconds(), 512);
     }
 
+    #[test]
+    fn from_512_second_intervals_roundtrip() {
+        let intervals = 100_u16;
+        let locktime = NumberOf512Seconds::from_512_second_intervals(intervals);
+        assert_eq!(locktime.to_512_second_intervals(), intervals);
+    }
+    
     #[test]
     fn from_seconds_ceil_success() {
         let actual = NumberOf512Seconds::from_seconds_ceil(100).unwrap();