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
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.