Simplify `Tunit::Test#skipped?`

If a `failure` is not present it, i.e it is `nil`, that will test
_never_ be skipped

     Exception === nil

Will be false always, so we are good
Move `skip` to `Tunit::Test`

- Add tests to it
- Refactor it by simplifying variable-names
Simplify `Test.order!`
Move `Test.test_order` to Runnable and rename to `Runnable.order`
Remove accessor to quell ruby warnings