nixos/test-driver: exit early if /dev/vhost-vsock isn't available (#406455)

nixos/doc/manual/development/running-nixos-tests-interactively.section.md

@@ -71,10 +71,19 @@ An SSH-based backdoor to log into machines can be enabled with
 {
   name = "…";
   nodes.machines = { /* … */ };
-  sshBackdoor.enable = true;
+  interactive.sshBackdoor.enable = true;
 }
 ```
 
+::: {.warning}
+Make sure to only enable the backdoor for interactive tests
+(i.e. by using `interactive.sshBackdoor.enable`)! This is the only
+supported configuration.
+
+Running a test in a sandbox with this will fail because `/dev/vhost-vsock` isn't available
+in the sandbox.
+:::
+
 This creates a [vsock socket](https://man7.org/linux/man-pages/man7/vsock.7.html)
 for each VM to log in with SSH. This configures root login with an empty password.
 

nixos/lib/testing/run.nix

@@ -43,7 +43,10 @@ in
   };
 
   config = {
-    rawTestDerivation = hostPkgs.stdenv.mkDerivation {
+    rawTestDerivation =
+      assert lib.assertMsg (!config.sshBackdoor.enable)
+        "The SSH backdoor is currently not supported for non-interactive testing! Please make sure to only set `interactive.sshBackdoor.enable = true;`!";
+      hostPkgs.stdenv.mkDerivation {
         name = "vm-test-run-${config.name}";
 
         requiredSystemFeatures =