nixos/tests/gnome3.nix: Speed up test

It turns out that "journalctl -f | grep -m 1 pattern" will block for
one more line after "pattern" appears, which can take a long time.

Changed files
+1 -1
nixos
tests
+1 -1
nixos/tests/gnome3.nix
···
$machine->succeed("su - alice -c 'DISPLAY=:0.0 gnome-terminal &'");
$machine->succeed("xauth merge ~alice/.Xauthority");
$machine->waitForWindow(qr/Terminal/);
-
$machine->mustSucceed("timeout 900 bash -c 'journalctl -f|grep -m 1 \"GNOME Shell started\"'");
+
$machine->succeed("timeout 900 bash -c 'while read msg; do if [[ \$msg =~ \"GNOME Shell started\" ]]; then break; fi; done < <(journalctl -f)'");
$machine->sleep(10);
$machine->screenshot("screen");
'';