This one fails for me because my test system doesn't have sudo
installed.  I can't see any reason why a simple su wouldn't be enough.

We already have a helper to make su usable on both IRIX and Linux in
test 123, and it might be a good idea to add this to the common routines
and use it.

I'd also suggest to split this test off 252 into a new test case, as
unlike the other tests it actually requires a scratch devices, and in
general is pretty different from the simple xfs_io exercises in 252.

I'd also move the newly added helpers directly into the new testcase
as they really aren't common.