configure only once per test

In `master` all the tests which call `driver.configure` more than once are failing. This PR fixes that.

Calling `driver.configure` will [create a new buffer](https://github.com/fluent/fluentd/blob/f4d1cd86cc0ba9a42b0d962a60c9b82a4e6ea286/lib/fluent/output.rb#L186-L187) and overwrite any previous settings.