wallet: SelectCoins, return early if wallet's UTXOs cannot cover the target

The CoinsResult class will now count the raw total amount and the effective
total amount internally (inside the 'CoinsResult::Add' and 'CoinsResult::Erase'
methods).
So there is no discrepancy between what we add/erase and the total values.
(which is what was happening on the coinselector_test because the 'CoinsResult'
object is manually created there, and we were not keeping the total amount
in sync with the outputs being added/removed).

src/wallet/spend.h

@@ -51,8 +51,10 @@ struct CoinsResult {
     void Shuffle(FastRandomContext& rng_fast);
     void Add(OutputType type, const COutput& out);
 
-    /** Sum of all available coins */
+    /** Sum of all available coins raw value */
     CAmount total_amount{0};
+    /** Sum of all available coins effective value (each output value minus fees required to spend it) */
+    std::optional<CAmount> total_effective_amount{0};
 };
 
 struct CoinFilterParams {