descriptor: ToPrivateString() pass if at least 1 priv key exists

- Refactor Descriptor::ToPrivateString() to allow descriptors with
  missing private keys to be printed. Useful in descriptors with
  multiple keys e.g tr() etc.
- The existing behaviour of listdescriptors is preserved as much as
  possible, if no private keys are availablle ToPrivateString will
  return false

src/test/descriptor_tests.cpp

@@ -49,7 +49,7 @@ constexpr int SIGNABLE = 1 << 3; // We can sign with this descriptor (this is no
 constexpr int DERIVE_HARDENED = 1 << 4; // The final derivation is hardened, i.e. ends with *' or *h
 constexpr int MIXED_PUBKEYS = 1 << 5;
 constexpr int XONLY_KEYS = 1 << 6; // X-only pubkeys are in use (and thus inferring/caching may swap parity of pubkeys/keyids)
-constexpr int MISSING_PRIVKEYS = 1 << 7; // Not all private keys are available. ToPrivateString() will fail and HavePrivateKeys() will return `false`.
+constexpr int MISSING_PRIVKEYS = 1 << 7; // Not all private keys are available. ToPrivateString() will return true if there is at least one private key and HavePrivateKeys() will return `false`.
 constexpr int SIGNABLE_FAILS = 1 << 8; // We can sign with this descriptor, but actually trying to sign will fail
 constexpr int MUSIG = 1 << 9; // This is a MuSig so key counts will have an extra key
 constexpr int MUSIG_DERIVATION = 1 << 10; // MuSig with BIP 328 derivation from the aggregate key
@@ -264,6 +264,12 @@ void DoCheck(std::string prv, std::string pub, const std::string& norm_pub, int
         parse_pub->ExpandPrivate(0, keys_priv, pub_prov);
 
         BOOST_CHECK_MESSAGE(EqualSigningProviders(priv_prov, pub_prov), "Private desc: " + prv + " Pub desc: " + pub);
+    } else if (keys_priv.keys.size() > 0) {
+        // If there is at least one private key, ToPrivateString() should return true and include that key
+        std::string prv_str;
+        BOOST_CHECK(parse_priv->ToPrivateString(keys_priv, prv_str));
+        size_t checksum_len = 9; // Including the '#' character
+        BOOST_CHECK_MESSAGE(prv == prv_str.substr(0, prv_str.length() - checksum_len), prv);
     }
 
     // Check that private can produce the normalized descriptors