nixos/tests/switch-test: Ensures the test fails on failure (#55744)

The `| tee` invocation always masked the return value of the
switch-to-configuration test.

```
~ $ false | tee && echo "oh no"
oh no
```

The added wrapper script will still output everything to stderr, while
passing failures to the test harness.

1 files changed, 11 insertions, 2 deletions

diff --git a/nixos/tests/switch-test.nix b/nixos/tests/switch-test.nix
index 32010838e67b4..0dba3697980f9 100644
--- a/nixos/tests/switch-test.nix
+++ b/nixos/tests/switch-test.nix
@@ -18,8 +18,17 @@ import ./make-test.nix ({ pkgs, ...} : {
   testScript = {nodes, ...}: let
     originalSystem = nodes.machine.config.system.build.toplevel;
     otherSystem = nodes.other.config.system.build.toplevel;
+
+    # Ensures failures pass through using pipefail, otherwise failing to
+    # switch-to-configuration is hidden by the success of `tee`.
+    stderrRunner = pkgs.writeScript "stderr-runner" ''
+      #! ${pkgs.stdenv.shell}
+      set -e
+      set -o pipefail
+      exec env -i "$@" | tee /dev/stderr
+    '';
   in ''
-    $machine->succeed("env -i ${originalSystem}/bin/switch-to-configuration test | tee /dev/stderr");
-    $machine->succeed("env -i ${otherSystem}/bin/switch-to-configuration test | tee /dev/stderr");
+    $machine->succeed("${stderrRunner} ${originalSystem}/bin/switch-to-configuration test");
+    $machine->succeed("${stderrRunner} ${otherSystem}/bin/switch-to-configuration test");
   '';
 })