an 'a' partition covering the whole disk, instead use the raw partition.
Also skip this test if the fss device is not available.
With lots of help from pgoyette.
XXX For now, we just create required files (including mount-points)
XXX in the test's working directory. Eventually someone with more
XXX rump-foo than I should create a fss rump-component and then update
XXX the test to run under rump.
martin