sdjournal: add test for NewJournalFromDir()
sdjournal: add test for NewJournalFromDir()

Simple test to exercise main code path and error handling of
`NewJournalFromDir()`.

endocode/go-systemd,coreos/go-systemd,coreos/go-systemd,glerchundi/go-systemd,jonboulle/go-systemd,jonboulle/go-systemd,endocode/go-systemd,glerchundi/go-systemd