nixos/xsslock: wait until `pgrep xlock` succeeds (#40810)

Please refer to the conversation in the original PR (#40619) for further
reference: https://github.com/NixOS/nixpkgs/pull/40619#issuecomment-390437845

It takes some time after the standby to trigger the locker which might
break the test on Hydra.

Changed files
+1 -1
nixos
tests
+1 -1
nixos/tests/xss-lock.nix
···
$machine->fail("pgrep xlock");
$machine->succeed("su -l alice -c 'xset dpms force standby'");
-
$machine->succeed("pgrep xlock");
'';
})
···
$machine->fail("pgrep xlock");
$machine->succeed("su -l alice -c 'xset dpms force standby'");
+
$machine->waitUntilSucceeds("pgrep xlock");
'';
})