xfstests should be skipped if no available scratch drives

[Po-Hsu Lin]

This issue was spotted on KVM kernels, the xfstests is now going to be tested because we have switched to the generic kernel test suite list from the cloud test list for it.

And it will fail for sure, as it needs a scratch drive to run.

In KT, we have a:
   'scratch' : True,
requirement in the lib/testprops.py

Combined with the:
    scratch drive: "/dev/sdb"
configuration in lib/lab-configuration.yaml we know on which node this test could be executed.

Need to check if we have this or other similar feature implemented in CKCT.
Or figure out a solution for this.