Goodbye RSpec, long live Test::Unit

Using test-unit 3.0 instead of RSpec.

Some specs are work in progress.
It is not related with unit-test actually, but using `-` would be better like:

```
     root = Fluent::Config::DSL::Parser.parse(<<-DSL)
        blah blah
     DSL
```

because we can indent the termination keyword (`DSL`).