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()
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors