It is my understanding that setting an environment varialbe `TEST_AWS_ACCOUNT_ID` for localstack sets that as the AWS account for the user in localstack.

However, setting it does not actually change any ARNs. For example:

1. Start localstack using `TEST_AWS_ACCOUNT_ID="111111111111" DEBUG=1 localstack start`
2. Deploy anything with an ARN (in this example I deployed a lambda)
3. Check the ARN. Example output for lambda:
```json
$ awslocal lambda list-functions
{
    "Functions": [
        {
            "FunctionName": "localtest-local-hello",
            "FunctionArn": "arn:aws:lambda:us-east-1:000000000000:function:localtest-local-hello",
            "Runtime": "nodejs10.x",
            "Role": "test_role",
            "Handler": "handler.hello",
            "CodeSize": 3077,
            "Description": "",
            "Timeout": 6,
            "MemorySize": 1024,
            "LastModified": "2019-11-06T06:43:53.451+0000",
            "CodeSha256": "V77gR13o90ET7+XquXBBc3Rxr1qDu1bE7S+A0Wbx5ys=",
            "Version": "$LATEST",
            "TracingConfig": {
                "Mode": "PassThrough"
            },
            "RevisionId": "3d4ed679-c1a9-4b7e-ad9e-f27e8543c5e9"
        }
    ]
}
```

ARN is still 000000000000
parsed_result = common.json_safe(parsed_result)
            response.status_code = int(parsed_result.get('statusCode', 200))
            response.headers.update(parsed_result.get('headers', {}))
            try:
                if isinstance(parsed_result['body'], dict):
                    response._content = json.dumps(parsed_result['body'])
                else:
                    response._content = to_bytes(parsed_result['body'])
            except Exception:
                response._content = '{}'
            response.headers['Content-Length'] = len(response._content)
            return response
        else:
            msg = 'API Gateway action uri "%s" not yet implemented' % uri
            LOGGER.warning(msg)
            return make_error(msg, 404)
    elif integration['type'] == 'HTTP':
        function = getattr(requests, method.lower())
        if isinstance(data, dict):
            data = json.dumps(data)
        result = function(integration['uri'], data=data, headers=headers)
        return result
    else:
        msg = ('API Gateway integration type "%s" for method "%s" not yet implemented' %
               (integration['type'], method))
        LOGGER.warning(msg)
        return make_error(msg, 404)
    return 200
UPDATE_APIGATEWAY = ProxyListenerApiGateway()