ctl: skip the test if ctladm is missing (WITHOUT_ISCSI)

Reviewed by: asomers
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D46853

ctl: add tests for START STOP UNIT

MFC after: 2 weeks
Sponsored by: Axcient
Reviewed by: emaste, markj
Pull Request: https://github.com/freebsd/freebsd-src/pull/1409