nixos/tests/systemd-journal: ensure that --grep works

Previously, after the version bump to v247, we broke journalctl --grep
as libpcre2 was lazily loaded during runtime using dlopen(3). This
ensures that we have a test case that alerts us when it fails again.

nixos/tests/systemd-journal.nix

@@ -13,6 +13,8 @@ import ./make-test-python.nix ({ pkgs, ... }:
   testScript = ''
     machine.wait_for_unit("multi-user.target")
 
+    machine.succeed("journalctl --grep=systemd")
+
     machine.succeed(
         "${pkgs.curl}/bin/curl -s localhost:19531/machine | ${pkgs.jq}/bin/jq -e '.hostname == \"machine\"'"
     )