Add some basic tests for hexdump(1)'s various output flags. Formatting
tests are omitted for this initial run as there are still some bugs to work
out there.

This covers -s flag testing on devices and non-devices that would have
caught breakage found in PR 219173 as well as other subtle breakage caused
locally.

Reviewed by: cem, ngie
Approved by: cem (acting co-mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D11279