ssh/tailssh: add more SSH tests, blend in env from ssh session

Updates #3802

Change-Id: I568c661cacbb0524afcd8be9577457ddba611f19
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ssh/tailssh/tailssh.go

@@ -232,11 +232,11 @@ func (srv *server) handleAcceptedSSH(ctx context.Context, s ssh.Session, ci *ssh
 		}
 	}
 	cmd.Dir = lu.HomeDir
+	cmd.Env = append(cmd.Env, s.Environ()...)
 	cmd.Env = append(cmd.Env, envForUser(lu)...)
 	if ptyReq.Term != "" {
 		cmd.Env = append(cmd.Env, fmt.Sprintf("TERM=%s", ptyReq.Term))
 	}
-	// TODO(bradfitz,maisem): also blend in user's s.Environ()
 	logf("Running: %q", cmd.Args)
 	var toCmd io.WriteCloser
 	var fromCmd io.ReadCloser