TextIteratorTest: Add a utility function to reduce the amount of boilerplate.

A trivial clean-up of unit tests for TextIterator.

BUG=none

Review URL: https://codereview.chromium.org/105323003

git-svn-id: svn://svn.chromium.org/blink/trunk@163387 bbb929c8-8fbe-4397-9dbb-9b2b20218538