wallet: check solvability using descriptor in AvailableCoins

This is a workaround for Miniscript descriptors containing hash
challenges. For those we can't mock the signature creator without making
OP_EQUAL mockable in the interpreter, so CalculateMaximumInputSize will
always return -1 and outputs for these descriptors would appear
unsolvable while they actually are.

Author: darosior

src/wallet/spend.cpp

@@ -294,9 +294,7 @@ CoinsResult AvailableCoins(const CWallet& wallet,
             std::unique_ptr<SigningProvider> provider = wallet.GetSolvingProvider(output.scriptPubKey);
 
             int input_bytes = CalculateMaximumSignedInputSize(output, COutPoint(), provider.get(), coinControl);
-            // Because CalculateMaximumSignedInputSize just uses ProduceSignature and makes a dummy signature,
-            // it is safe to assume that this input is solvable if input_bytes is greater -1.
-            bool solvable = input_bytes > -1;
+            bool solvable = provider ? InferDescriptor(output.scriptPubKey, *provider)->IsSolvable() : false;
             bool spendable = ((mine & ISMINE_SPENDABLE) != ISMINE_NO) || (((mine & ISMINE_WATCH_ONLY) != ISMINE_NO) && (coinControl && coinControl->fAllowWatchOnly && solvable));
 
             // Filter by spendable outputs only