Skip cli_user tests when insufficient disks are available

tests/sys/cddl/zfs/tests/cli_user/misc/misc_test.sh
tests/sys/cddl/zfs/tests/cli_user/zpool_iostat/zpool_iostat_test.sh
tests/sys/cddl/zfs/tests/cli_user/zpool_list/zpool_list_test.sh
tests/sys/cddl/zfs/tests/cli_user/zfs_list/zfs_list_test.sh
Add appropriate verify_disk_count calls

tests/sys/cddl/zfs/tests/cli_user/misc/zpool_add_001_neg.ksh
tests/sys/cddl/zfs/tests/cli_user/misc/zpool_add_002_pos.ksh
Fix these tests so they won't attempt to add a disk that's already
in use.

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