AWS CDK Unittest不能模拟NetworkLoadBalancer资源，并且此问题会导致难以进行单元测试。为了解决这个问题，可以使用AWS CDK提供的CDKIntegrationTest工具和MockIntegrationTest构建一组集成测试来测试网络负载平衡器资源。以下是一个示例代码，该代码使用了CDKIntegrationTest和MockIntegrationTest：

import boto3
from aws_cdk import (
    core as cdk,
    aws_ec2 as ec2,
    aws_elasticloadbalancingv2 as elbv2,
    testing as cdk_testing
)

class MyStack(cdk.Stack):
    def __init__(self, scope: cdk.Construct, id: str, **kwargs) -> None:
        super().__init__(scope, id, **kwargs)

        vpc = ec2.Vpc(self, "MyVPC")

        alb = elbv2.NetworkLoadBalancer(
            self,
            "MyALB",
            vpc=vpc,
            internet_facing=False,
            load_balancer_name="MyLoadBalancer"
        )

        core.CfnOutput(self, "ALBARN", value=alb.load_balancer_arn)

class TestMyStack(unittest.TestCase):
    def test_stack(self):
        app = cdk.App()
        MyStack(app, "stack")

        # create a mock of the network load balancer
        mock_elbv2 = cdk_testing.MockAws(lambda: boto3.Session())
        mock_elbv2.add_mock("elbv2", "create_load_balancer", side_effect=Exception("cannot create network load balancer"))
        mock_elbv2.add_mock("elbv2", "delete_load_balancer", return_value={})

        # prepare the integration test
        integ_test = cdk_testing.CDKIntegrationTest(
            app,
            "test-stack",
            cloud_assembly_subdirectory="test-fixtures/test-stack.out"
        )
        # provide the mock to the integration test
        integ_test.set_aws(mock_elbv2.aws())

        # run the integration test and validate that the cloudformation stack was created successfully
        integ_test.synth()
        integ_test.deploy(expect_changes=True)

        stack = integ