Make group_utxos_if_applies group by pubkey

Now avoid_partial_spends will work as intended.
If passed as true, it will group utxos by pubkey,
and if selected, the grouped UTXOs will be used together.
Tests are added that check that new functionality works

Author: martinsaposnic

crates/wallet/src/test_utils.rs

@@ -119,6 +119,59 @@ fn new_funded_wallet(descriptor: &str, change_descriptor: Option<&str>) -> (Wall
     (wallet, tx1.compute_txid())
 }
 
+/// Return a fake wallet that has only reused outputs for testing.
+///
+/// The wallet contains two transactions with one output each, both paying to the same common
+/// script. The common script is derived from the first unused external address.
+pub fn get_wallet_with_only_reused_outputs(
+    descriptor: &str,
+    change_descriptor: Option<&str>,
+) -> (Wallet, (Txid, Txid)) {
+    // Create an empty wallet.
+    let params = if let Some(change_desc) = change_descriptor {
+        Wallet::create(descriptor.to_string(), change_desc.to_string())
+    } else {
+        Wallet::create_single(descriptor.to_string())
+    };
+
+    let mut wallet = params
+        .network(Network::Regtest)
+        .create_wallet_no_persist()
+        .expect("descriptors must be valid");
+
+    // Derive a common external address to be reused.
+    let common_addr = wallet.next_unused_address(KeychainKind::External).address;
+    let common_script = common_addr.script_pubkey();
+
+    // Fabricate transaction 1 with one output paying to the common script.
+    let tx1 = Transaction {
+        version: transaction::Version::ONE,
+        lock_time: absolute::LockTime::ZERO,
+        input: vec![], // No inputs since it's fabricated.
+        output: vec![TxOut {
+            value: Amount::from_sat(500_000),
+            script_pubkey: common_script.clone(),
+        }],
+    };
+
+    // Fabricate transaction 2 with one output paying to the common script.
+    let tx2 = Transaction {
+        version: transaction::Version::ONE,
+        lock_time: absolute::LockTime::ZERO,
+        input: vec![],
+        output: vec![TxOut {
+            value: Amount::from_sat(100_000),
+            script_pubkey: common_script,
+        }],
+    };
+
+    // Insert these fabricated transactions into the wallet.
+    insert_tx(&mut wallet, tx1.clone());
+    insert_tx(&mut wallet, tx2.clone());
+
+    (wallet, (tx1.compute_txid(), tx2.compute_txid()))
+}
+
 /// Return a fake wallet that appears to be funded for testing.
 ///
 /// The funded wallet contains a tx with a 76_000 sats input and two outputs, one spending 25_000