tests: Cover recent bug fixes

tests: Cover recent bug fixes

Add a new test 'handshake' that intentionally provokes a server error
during option negotiation, before falling back to NBD_OPT_ABORT to
end negotiation, to prove that the server is correctly allowing a
client to fall back to known options; thus covering two recent bug
fixes for a server sending the wrong length in an error reply, and
for a server not reading enough data when replying to an unknown
command.

Signed-off-by: Eric Blake <eblake@redhat.com>
[reflowed in light of b8852465a]
Signed-off-by: Wouter Verhelst <w@uter.be>