Test that requests with large request Content-Length cannot cause a denial-of-service

[mx-shift]

HTTP does not specify a limit on request body size but allows implementations to enforce one to allow bounding of resource usage. Even if the actual request body is small, direct usage of a client-supplied Content-Length header can cause excessive memory usage potentially leading to a denial-of-service. Dropshot has a config option to set a maximum request body size but no test to verify that that limit is enforced for this case.