Fix the preformatting in 4 more articles

This kind of piggybacks off of 8077a7d27ddac558aa012ea9a33d86f715fe6942, to fix a few more white-background pre tags:

* [Unit Testing](https://www.dartlang.org/articles/dart-unit-tests/) YAML file has no padding; white boxes
* [I/O](https://www.dartlang.org/articles/io/) has white boxes
* [Unit Tests for Pub](https://www.dartlang.org/articles/writing-unit-tests-for-pub-packages/) has white boxes
* [Native Extensions](https://www.dartlang.org/articles/native-extensions-for-standalone-dart-vm/) C++ files have no padding (like `void wrappedRandomArray`); white boxes

Staged here: https://fix-more-preformatted-dot-dart-lang.appspot.com