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.
This test allows to ensure the systemd-journal-gatewayd service is
responding correcly when the NixOS option `enableHttpGateway` is set.
The test has not been added into the main systemd test because a
graphical stack is not required (and rebuilding the graphical stack on
systemd change is huge).
