test cases to verify client behavior in the face of too-large response messages

[jhump]

We previously had a test suite named "Client Message Size", but it could not test what it needed to test due to limitations in the framework. (See relevant TODO here.)

They have since been removed. But we should address that TODO and add them back. Addressing the TODO will require some tweaks to the Protobuf request messages (to instruct the server to expand response sizes) and improvements in the reference server (to perform said expansion before sending response messages). We'll also need some adjustments in the test case assertions (that examine the client output and decide if a test case is successful) to take into account the expansion when deciding whether the response messages look correct.