Rework the way we parse the TestExpectations in webkitpy.

The logic for iterating over the list of expectations files
is a bit convoluted, perhaps because of the way the Test port
was set up. This patch cleans things up a bit to be more consistent.

R=ojan@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/22984011

git-svn-id: svn://svn.chromium.org/blink/trunk@156354 bbb929c8-8fbe-4397-9dbb-9b2b20218538