nixos tests: waitForWindow: output a list of windows we see prior to the final check

machine: must succeed: xwininfo -root -tree | sed 's/.*0x[0-9a-f]* \"\([^\"]*\)\".*/\1/; t; d'
machine: exit status 0
machine: Last chance to match /(?^:dfiirst configuration)/ on the the window list, which currently contains:
machine: [i3 con] container around 0xf8a5f0, i3: first configuration, [i3 con] floatingcon around 0xf8c260, [i3 con] container around 0xf8a380, i3bar for output Virtual-1, [i3 con] bottom dockarea Virtual-1, [i3 con] workspace 1, [i3 con] content Virtual-1, [i3 con] top dockarea Virtual-1, [i3 con] output Virtual-1, [i3 con] workspace __i3_scratch, [i3 con] content __i3, [i3 con] pseudo-output __i3, i3

nixos/lib/test-driver/Machine.pm

···
     $self->nest("waiting for a window to appear", sub {
         retry sub {
             my @names = $self->getWindowNames;
+
+            my ($retries_remaining) = @_;
+            if ($retries_remaining == 0) {
+                $self->log("Last chance to match /$regexp/ on the the window list, which currently contains:");
+                $self->log(join(", ", @names));
+            }
+
             foreach my $n (@names) {
                 return 1 if $n =~ /$regexp/;
             }