Skip test when filesystem does not have xattr

Addresses regression from a104944 seen on CloudBees.
I'm fine with doing this here, but would it be "more correct" _not_ to do this in a setup for a specific test, but to have a `TestUtils.filesystemSupportsXattr()` method that would carry out this check outside of the scope of any test?

Then the `SkipException` could still happen _at the beginning_ of the test, which is where it seems it should go..?