sd_journal_send() passes an entry that doesn't fit into a datagram to
journald as a sealed memfd. Only test-journal-send makes a real journald
receive a memfd. It does so by sending a 4 MiB message to the journal of
the machine it runs on.
Let's send a sealed memfd with a 4 MiB field to journald in
TEST-04-JOURNAL and check that journalctl shows the whole field. Then
test-journal-send can test the memfd without writing to the journal of
the host.