MFC r320930,r324286: Add tests for hexdump(1)

MFC r320930:
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.

MFC r324286: hexdump: Remove expected test failures for now succeeding tests

r323990 fixed the -s flag breakage reported by PR 219173. Mark the
corresponding hexdump tests accordingly, since they should now both succeed.

PR: 222781

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