nixos: tests: firefox: make more comprehensive

Run Firefox inside an XTerm, it doesn't crash mysteriously this way.
Also try opening developer tools and checking that Firefox doesn't
crash in the process.

nixos/tests/firefox.nix

@@ -8,15 +8,21 @@ import ./make-test.nix ({ pkgs, ... }: {
     { config, pkgs, ... }:
 
     { imports = [ ./common/x11.nix ];
-      environment.systemPackages = [ pkgs.firefox ];
+      environment.systemPackages = [ pkgs.firefox pkgs.xdotool ];
     };
 
   testScript =
     ''
       $machine->waitForX;
-      $machine->execute("firefox file://${pkgs.valgrind.doc}/share/doc/valgrind/html/index.html &");
+      $machine->execute("xterm -e 'firefox file://${pkgs.valgrind.doc}/share/doc/valgrind/html/index.html' &");
       $machine->waitForWindow(qr/Valgrind/);
       $machine->sleep(40); # wait until Firefox has finished loading the page
+      $machine->execute("xdotool key space"); # do I want to make Firefox the
+                             # default browser? I just want to close the dialog
+      $machine->sleep(2); # wait until Firefox hides the default browser window
+      $machine->execute("xdotool key F12");
+      $machine->sleep(10); # wait until Firefox draws the developer tool panel
+      $machine->succeed("xwininfo -root -tree | grep Valgrind");
       $machine->screenshot("screen");
     '';