Add unit test for deserialize non-standard sighash

It is possible, although not immediately obvious, that it is possible to
create a `PsbtSigHashType` with a non-standard value.

Add a unit test to show this and also catch any regressions if we
accidental change this logic.

src/util/psbt/serialize.rs

@@ -367,3 +367,15 @@ impl Deserialize for TapTree {
 fn key_source_len(key_source: &KeySource) -> usize {
     4 + 4 * (key_source.1).as_ref().len()
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn can_deserialize_non_standard_psbt_sig_hash_type() {
+        let non_standard_sighash = [222u8, 0u8, 0u8, 0u8]; // 32 byte value.
+        let sighash = PsbtSigHashType::deserialize(&non_standard_sighash);
+        assert!(sighash.is_ok())
+    }
+}