Disable read-only testing of space usage properties.

These properties (used, available, referenced) are easily influenced by
internal machinations in ZFS, and currently cause random failures of this
test when run on a system running the BP workers.

The test itself checks that the properties can't be changed, and goes
farther than just asserting that the 'zfs set' command fails: it also checks
that the property's value still matches its original value.

For some reason, however, background activity causes short-term changes in
these values, that appear not to involve either an ioctl (at least one that
would show up in 'zpool history') or filesystem accesses. This is hopefully
still only temporary just to get the tests passing.

Submitted by: Will
Sponsored by: Spectra Logic Corp