fix(abstract-utxo): add fee rate test for backup key recovery

Add test case to verify the actual fee rate used in backup key recovery
transactions. This ensures that the fee calculation logic is working
as expected.

Note that basically all tests fail because fee calculation is utterly broken.

Issue: BTC-2891

Co-authored-by: llm-git <[email protected]>

Author: OttoAllmendinger

modules/abstract-utxo/test/unit/recovery/backupKeyRecovery.ts

@@ -7,12 +7,14 @@ import nock = require('nock');
 import { BIP32Interface } from '@bitgo/utxo-lib';
 import * as utxolib from '@bitgo/utxo-lib';
 import { Config, krsProviders, Triple } from '@bitgo/sdk-core';
+import { Dimensions } from '@bitgo/unspents';
 
 import {
   AbstractUtxoCoin,
   backupKeyRecovery,
   BackupKeyRecoveryTransansaction,
   CoingeckoApi,
+  DEFAULT_RECOVERY_FEERATE_SAT_VBYTE_V2,
   FormattedOfflineVaultTxInfo,
 } from '../../../src';
 import {
@@ -190,6 +192,21 @@ function run(
       );
     });
 
+    it('has expected fee rate', function () {
+      if (!(recoveryTx instanceof utxolib.bitgo.UtxoPsbt)) {
+        this.skip();
+      }
+      const expectedFeeRate = params.feeRate ?? DEFAULT_RECOVERY_FEERATE_SAT_VBYTE_V2;
+      const inputSum = utxolib.bitgo.unspentSum(recoverUnspents, 'bigint');
+      const outputSum = recoveryTx.txOutputs.reduce((sum, o) => sum + o.value, BigInt(0));
+      const fee = inputSum - outputSum;
+      const vsize = Dimensions.fromPsbt(recoveryTx).getVSize();
+      const feeRateSatB = Number(fee) / vsize;
+      const diff = Math.abs(feeRateSatB - expectedFeeRate) / expectedFeeRate;
+      // within 10%
+      assert.strictEqual(diff < 0.1, true, `expected fee rate ${expectedFeeRate} but got ${feeRateSatB}`);
+    });
+
     it('matches fixture', async function () {
       const fixtureCoin = getNormalTestnetCoin(coin);
       const fixtureRecovery = { ...recovery };