MFC r359717:

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.

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