op-node: testutils RPC error faker util (#3599)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

op-node/testutils/rpc_err_faker.go

+package testutils
+
+import (
+	"context"
+
+	"github.com/ethereum/go-ethereum/rpc"
+
+	"github.com/ethereum-optimism/optimism/op-node/client"
+)
+
+// RPCErrFaker implements an RPC by wrapping one, but returns an error when prepared with one, to test RPC error handling.
+type RPCErrFaker struct {
+	// RPC to call when no ErrFn is set, or the ErrFn does not return an error
+	RPC client.RPC
+	// ErrFn returns an error when the RPC needs to return error upon a call, batch call or subscription.
+	// The RPC operates without fake errors if the ErrFn is nil, or returns nil.
+	ErrFn func() error
+}
+
+func (r RPCErrFaker) Close() {
+	r.RPC.Close()
+}
+
+func (r RPCErrFaker) CallContext(ctx context.Context, result interface{}, method string, args ...interface{}) error {
+	if r.ErrFn != nil {
+		if err := r.ErrFn(); err != nil {
+			return err
+		}
+	}
+	return r.RPC.CallContext(ctx, result, method, args...)
+}
+
+func (r RPCErrFaker) BatchCallContext(ctx context.Context, b []rpc.BatchElem) error {
+	if r.ErrFn != nil {
+		if err := r.ErrFn(); err != nil {
+			return err
+		}
+	}
+	return r.RPC.BatchCallContext(ctx, b)
+}
+
+func (r RPCErrFaker) EthSubscribe(ctx context.Context, channel interface{}, args ...interface{}) (*rpc.ClientSubscription, error) {
+	if r.ErrFn != nil {
+		if err := r.ErrFn(); err != nil {
+			return nil, err
+		}
+	}
+	return r.RPC.EthSubscribe(ctx, channel, args...)
+}
+
+var _ client.RPC = (*RPCErrFaker)(nil)