Code review comment for ~raharper/curtin:fix/support-i386

I think doing it in the base class protects against when we add new base-class tests that would fail, we'd need to add a skip in the zfsroot class which could have copied all of the base class tests and decorated them.

I'm open to alternatives that are lighter if you have them; and we may want to create new expected_failure classes to test error paths, in which case we'd want something like this anyhow.