Reviewed by Ryosuke Niwa.

        Refactor the extend-selection tests to be clearer what they're doing
        https://bugs.webkit.org/show_bug.cgi?id=54429

        This is essentially a cleanup to extend-selection.js and propagation of those changes to the affected tests.

        * editing/selection/extend-selection-character.html:
        * editing/selection/extend-selection-enclosing-block.html:
        * editing/selection/extend-selection-home-end.html:
        * editing/selection/extend-selection-word.html:
        * editing/selection/resources/extend-selection.js:
        (getSerializedSelection):
        (extendSelectionWithinBlock):
        (extendSelectionToEnd):
        (fold):
        (logMismatchingPositions):
        (extendAndLogSelection):
        (extendAndLogSelectionWithinBlock):
        (extendAndLogSelectionToEnd):
        (runSelectionTestsWithGranularity):
        (getTestNodeContainer):
        (createNode):
        (createCharAndWordNodes):
        (createEnclosingBlockNodes):
        (createHomeEndNodes):
        (createAllNodes):
        (.window.onload):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@78565 268f45cc-cd09-0410-ab3c-d52691b4dbfc