Test that max request body size is enforced for chunked encoding

[mx-shift]

HTTP does not specify a limit on request body size but allows implementations to enforce one to allow bounding of resource usage. When chunked encoding is used, the request body length is not known upfront and is only known incrementally as each chunk is received. Dropshot has a config option to set a maximum request body size but no test to verify that that limit is enforced when chunked encoding is used and a steady stream of chunks, each individually smaller than the limit, are received.

[sunshowers]

This might interact with #542 -- I believe the way it's done today and in #542 makes it so that we check the actual request length rather than just relying on what the header says.

Still most likely need a test for this though!