watchlist: Add a way to load the watchlist from config.

watchlist: Add a way to load the watchlist from config.
https://bugs.webkit.org/show_bug.cgi?id=68991

Reviewed by Eric Seidel.

* Scripts/webkitpy/common/config/watchlist: Added.
* Scripts/webkitpy/common/watchlist/watchlistloader.py: Added.
Support for loading the watch list.
* Scripts/webkitpy/common/watchlist/watchlistloader_unittest.py: Added.
Tests for the loading.
* Scripts/webkitpy/common/watchlist/watchlistparser_unittest.py:
Adjusted to use the common base class.
* Scripts/webkitpy/common/webkitunittest.py: Added.
Made a common base class for functionality used in more than one.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@96263 268f45cc-cd09-0410-ab3c-d52691b4dbfc