pow: Unit test from_hex_internal

Add a unit test that fails if put before the "pow: Fix off-by-one error"
patch. Tests that we can correctly parse a 32 character long hex string
into a `U256`.

bitcoin/src/pow.rs

@@ -1652,6 +1652,14 @@ mod tests {
         assert_eq!(got, val);
     }
 
+    #[test]
+    fn u256_from_hex_32_characters_long() {
+        let hex = "a69b455cd41bb662a69b4555deadbeef";
+        let want = U256(0x00, 0xA69B_455C_D41B_B662_A69B_4555_DEAD_BEEF);
+        let got = U256::from_unprefixed_hex(hex).expect("failed to parse hex");
+        assert_eq!(got, want);
+    }
+
     #[cfg(feature = "serde")]
     #[test]
     fn u256_serde() {