ssh/tailssh: add exit-status test harness

Extract the in-process SSH server harness out of tailssh_test.go into
tailssh_exitcodes_test.go so the exit-status tests can share it, and add
the integrationtest-tagged exit-code suite (Go and OpenSSH clients).

Updates #18256

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>