File assertions: hasParent, hasExtension, hasName

I needed some assertions on File recently that I didn't find so I quickly write them.
Are you interested by a PR ?

* `hasParent(File expected)` verifies that a File has expected as a parent (File#getParentFile)
* `hasExtension(String extension)` verifies that a File has the expected file extension
* `hasName(String name)` verifies that a File has the expected name (File#getName)