test/sandbox: check mount outcome

Do this at the beginning of the test for early failure.

Signed-off-by: Ophestra <cat@gensokyo.uk>

test/configuration.nix

@@ -102,6 +102,21 @@
     home-manager = _: _: { home.stateVersion = "23.05"; };
 
     apps = [
+      {
+        name = "check-sandbox";
+        verbose = true;
+        share = pkgs.foot;
+        packages = [ ];
+        command = "${pkgs.callPackage ./sandbox {
+          inherit (config.environment.fortify.package) version;
+        }}";
+        extraPaths = [
+          {
+            src = "/proc/mounts";
+            dst = "/.fortify/host-mounts";
+          }
+        ];
+      }
       {
         name = "ne-foot";
         verbose = true;