nixos X tests: wait for logind to link a session to the server

There seems to be some race causing failures if an X command gets in before slim starts the session

Signed-off-by: Shea Levy <shea@shealevy.com>

nixos/lib/test-driver/Machine.pm

@@ -495,6 +495,8 @@ sub waitForX {
     my ($self, $regexp) = @_;
     $self->nest("waiting for the X11 server", sub {
         retry sub {
+            my ($status, $out) = $self->execute("journalctl -bu systemd-logind | grep Linked");
+            return 0 if $status != 0;
             my ($status, $out) = $self->execute("xwininfo -root > /dev/null 2>&1");
             return 1 if $status == 0;
         }

nixos/tests/firefox.nix

@@ -16,7 +16,6 @@
       $machine->waitForWindow(qr/Valgrind/);
       $machine->sleep(40); # wait until Firefox has finished loading the page
       $machine->screenshot("screen");
-
     '';
 
 }

nixos/tests/xfce.nix

@@ -17,6 +17,7 @@
 
   testScript =
     ''
+      $machine->waitForX;
       $machine->waitForWindow(qr/xfce4-panel/);
       $machine->sleep(10);