itest: skip error assertion when parent context finishes

We may get a flake like the following,
```
lnd_route_blinding_test.go:468:
            Error Trace:    /Users/runner/work/lnd/lnd/itest/lnd_route_blinding_test.go:468
                                        /Users/runner/hostedtoolcache/go/1.22.3/arm64/src/runtime/asm_arm64.s:1222
            Error:          Received unexpected error:
                            rpc error: code = Canceled desc = context canceled
            Test:           TestLightningNetworkDaemon/tranche15/144-of-156/bitcoind/disable_introduction_node
```

This happens when the test successfully finishes, the parent context is
canceled, causing the child context to return an error. We fix it by
ignoring it in the goroutine.

itest/lnd_route_blinding_test.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"crypto/sha256"
 	"encoding/hex"
+	"errors"
 	"time"
 
 	"github.com/btcsuite/btcd/btcec/v2"
@@ -465,6 +466,14 @@ func (b *blindedForwardTest) sendBlindedPayment(ctx context.Context,
 	ctx, cancel := context.WithTimeout(ctx, time.Hour)
 	go func() {
 		_, err := b.ht.Alice.RPC.Router.SendToRouteV2(ctx, sendReq)
+
+		// We may get a context canceled error when the test is
+		// finished.
+		if errors.Is(err, context.Canceled) {
+			b.ht.Logf("sendBlindedPayment: parent context canceled")
+			return
+		}
+
 		require.NoError(b.ht, err)
 	}()