loop/test: simplify preimage push test to be less dependent on height

Our preimage push test previously relied on our dropping down to the
default sweep conf target to mock a drop in chain fees. This makes
our test dependent on height, which makes changes to our sweep logic
regarding when we reveal our preimage break this test. In this commit
that logic is replaced with simply locking our mock and updating fees
on the fly.

test/lnd_services_mock.go

@@ -258,5 +258,5 @@ func (s *LndMockServices) DecodeInvoice(request string) (*zpay32.Invoice,
 func (s *LndMockServices) SetFeeEstimate(confTarget int32,
 	feeEstimate chainfee.SatPerKWeight) {
 
-	s.WalletKit.(*mockWalletKit).feeEstimates[confTarget] = feeEstimate
+	s.WalletKit.(*mockWalletKit).setFeeEstimate(confTarget, feeEstimate)
 }

test/walletkit_mock.go

@@ -3,6 +3,7 @@ package test
 import (
 	"context"
 	"errors"
+	"sync"
 	"time"
 
 	"github.com/btcsuite/btcd/chaincfg"
@@ -21,9 +22,11 @@ import (
 var DefaultMockFee = chainfee.SatPerKWeight(10000)
 
 type mockWalletKit struct {
-	lnd          *LndMockServices
-	keyIndex     int32
-	feeEstimates map[int32]chainfee.SatPerKWeight
+	lnd      *LndMockServices
+	keyIndex int32
+
+	feeEstimateLock sync.Mutex
+	feeEstimates    map[int32]chainfee.SatPerKWeight
 }
 
 var _ lndclient.WalletKitClient = (*mockWalletKit)(nil)
@@ -118,9 +121,19 @@ func (m *mockWalletKit) SendOutputs(ctx context.Context, outputs []*wire.TxOut,
 	return &tx, nil
 }
 
+func (m *mockWalletKit) setFeeEstimate(confTarget int32, fee chainfee.SatPerKWeight) {
+	m.feeEstimateLock.Lock()
+	defer m.feeEstimateLock.Unlock()
+
+	m.feeEstimates[confTarget] = fee
+}
+
 func (m *mockWalletKit) EstimateFee(ctx context.Context, confTarget int32) (
 	chainfee.SatPerKWeight, error) {
 
+	m.feeEstimateLock.Lock()
+	defer m.feeEstimateLock.Unlock()
+
 	if confTarget <= 1 {
 		return 0, errors.New("conf target must be greater than 1")
 	}