Adds a test class covering functions.compat.php

The functions.compat.php file has two functions that parse some
specific pieces of possible YouTube urls. This commit adds a test
class that tests each kind of sanitization that youtube_sanitize_url
performs and also test the two types of urls that jetpack_get_youtube_id
returns ids from.
Merge pull request #972 from enkrates/add-youtube-functions-test-class

Adds a test class covering functions.compat.php