Use 4x parallelism in unit tests

git-svn-id: http://zxing.googlecode.com/svn/trunk@2999 59b500cc-1b3d-0410-9834-0bbf25fbcc57
Handle smaller images in WhiteRectangleDetector

git-svn-id: http://zxing.googlecode.com/svn/trunk@3000 59b500cc-1b3d-0410-9834-0bbf25fbcc57