tests: add low-level SSH protocol tests We don't really have good low-level tests for the behavior of the SSH wire protocol. This commit attempts to establish some. The added tests consist of a mixture of starting a server with `hg serve --stdio` and sending bytes to it and using `hg debugpeer` to go through the official client code. Having insight into what raw bytes are exchanged as well as what the peer does is useful. We also introduce a test extension for modifying the behavior of the SSH server and peer. For example, we change the server to not recognize the "hello" command, simulating behavior of <0.9.1 servers. These tests are generally useful to have. But the impetus for creating them now is they will be needed for verifying behavior of old clients and servers when a new SSH protocol is introduced. Differential Revision: https://phab.mercurial-scm.org/D2026 # no-check-commit because of serve_forever()