Add a basic test for nvmecontrol

I recently made some bug fixes in nvmecontrol. It occurred to me that
since nvmecontrol lacks any kyua tests, I should convert the informal
testing I did into a more formal automated test. The test in this
change should be considered just a starting point; it is neither
complete nor thorough. While converting the test to ATF/kyua, I
discovered a small bug in nvmecontrol; the nvmecontrol devlist command
would always exit with an unsuccessful status. So I included the fix
for that, too, so that the test won't fail.

Reviewed by: imp@
MFC after: 3 days
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D24269