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
Break up method-chaining

Try having only 1 method call per line
Rename colliding variable `method` to `method_name`
Add `Expect#not_to` negate `Expect#to`