Update TestResourceHandler, UsingDir, UsingFile, FileHandlerSpec

* TestResourceHandler: Added new abstract function prepare and using it in function test. Now dirs and files are automatically created by test, no need to call create functions in the right order.
* UsingDir: Implements prepare. Added new public val dirs.
* UsingDir: Implements prepare. Added new public val files.
* FileHandlerSpec: Adapted to these changes.