Test secret hash truncation

When using `hashes` to hash a secret and print it in `Debug` we truncate
the hash into an 8 byte identifier, test it.

src/key.rs

@@ -1718,6 +1718,31 @@ mod test {
             "0100000000000000020000000000000003000000000000000400000000000000"
         );
     }
+    #[test]
+    fn test_display_obfuscation() {
+        #[rustfmt::skip]
+        static SK_BYTES: [u8; 32] = [
+            0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01,
+            0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
+            0xff, 0xff, 0x00, 0x00, 0xff, 0xff, 0x00, 0x00,
+            0x63, 0x63, 0x63, 0x63, 0x63, 0x63, 0x63, 0x63,
+        ];
+
+        let sk = SecretKey::from_slice(&SK_BYTES).expect("sk");
+
+        #[cfg(feature = "std")]
+        {
+            let got = format!("{:?}", sk);
+            let want = "SecretKey(#2f4d520bd6874292)";
+            assert_eq!(got, want);
+        }
+        #[cfg(all(not(feature = "std"), feature = "hashes"))]
+        {
+            let got = format!("{:?}", sk);
+            let want = "SecretKey(#de5a451f70110720)";
+            assert_eq!(got, want);
+        }
+    }
 
     #[test]
     #[cfg(feature = "alloc")]