Do not send response to CloudFormation when testing with sam local

When testing my custom resource with _sam local_, the resource helper tries to send a response to CloudFormation.

It generates an error since I am using fake information in my request payload.

Example:
```
{
"RequestType": "Create",
"ResponseURL": "http://pre-signed-S3-url-for-response",
"StackId": "arn:aws:cloudformation:us-east-1:123456789012:stack/MyStack/guid",
"RequestId": "unique id for this create request",
"ResourceType": "Custom::TestResource",
"LogicalResourceId": "MyTestResource",
...
```

It would be nice to disable response sending to cfn when testing with _sam local_.

crhelper version: 2.0.10

No file or test is named. Start by locating the custom resource helper's response-sending path and how local SAM requests are represented, then determine how local testing should disable the CloudFormation response. Done means local SAM testing no longer attempts the response and existing response behavior remains unchanged.

Written by the indexing model from the issue text.