syslogd: Fix tests when if_epair is not loaded

Take a cue from vnet.subr and abstract module loading, jail creation and
cleanup.

Note that the way jails are created differs slightly from vnet.subr, to
maintain the current style of the tests.

Reported by: Jenkins
Reviewed by: markj
Approved by: emaste (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48949