zfs: multiple improvements to the zpool_add tests

• Don't partition a disk if too few are available. Just rely on Kyua to ensure that the tests aren't run with insufficient disks.

• Remove redundant cleanup steps

• In zpool_add_003_pos, store the temporary file in $PWD so Kyua will automatically clean it up.

• Update zpool_add_005_pos to use dumpon instead of dumpadm. This test had never been ported to FreeBSD.

• In zpool_add_005_pos, don't format the dump disk with UFS. That was pointless.

MFC after: 2 weeks
Sponsored by: Axcient