Allow skipped zfstests
Allow skipped zfstests

Tests may be skipped if they are not supported for some reason
by the distribution.  The main use case for this is to skip
tests when partitionable loopback devices are unavailable.

Signed-off-by: Brian Behlendorf <32d50ad1e384dcb9a5bf7e40fa470df5dabaf348@llnl.gov>

zfsonlinux/zfs-buildbot,zfsonlinux/zfs-buildbot,gmelikov/zfs-buildbot,gmelikov/zfs-buildbot