Fix ZFS's unprivileged tests
The tests used to fail because the kyua process couldn't write to its own
log files after dropping privileges. This change fixes them by dropping
privileges only for the commands under test, so log file access will still
happen as root. A few tests (zpool_create_001_neg, zpool_create_002_pos,
zpool_iostat_003_neg, and zpool_list_002_neg) don't require any setup, so
they can run fully unprivileged.
Sponsored by: Spectra Logic Corp