nixos/testing: enable ssh backdoor by default if debug hook is enabled

You usually want both, so it makes sense to have the former imply the
latter. If this is not desired, it can still be modified within a test.

nixos/lib/testing/nodes.nix

@@ -84,7 +84,8 @@ in
   options = {
     sshBackdoor = {
       enable = mkOption {
-        default = false;
+        default = config.enableDebugHook;
+        defaultText = lib.literalExpression "config.enableDebugHook";
         type = types.bool;
         description = "Whether to turn on the VSOCK-based access to all VMs. This provides an unauthenticated access intended for debugging.";
       };