nixosTests.virtualbox: fix logging service

This used to be StandardOutput=syslog, which was removed because
syslog is deprecated, but that caused the test to fail.  So bring it
back, but set it to the non-deprecated "journal" value instead (which
is what systemd interprets "syslog" as now anyway).

Fixes: 962e15aebc ("nixos: remove StandardOutput=syslog, StandardError=syslog lines")

nixos/tests/virtualbox.nix

@@ -191,6 +191,7 @@ let
       systemd.services."vboxtestlog-${name}@" = {
         description = "VirtualBox Test Machine Log For ${name}";
         serviceConfig.StandardInput = "socket";
+        serviceConfig.StandardOutput = "journal";
         serviceConfig.SyslogIdentifier = "GUEST-${name}";
         serviceConfig.ExecStart = "${pkgs.coreutils}/bin/cat";
       };