Add unit test for existing StringBuilder

Add unit test for existing StringBuilder
https://bugs.webkit.org/show_bug.cgi?id=67080

Patch by Xianzhu Wang <wangxianzhu@chromium.org> on 2011-09-22
Reviewed by Darin Adler.

* TestWebKitAPI/TestWebKitAPI.gypi:
* TestWebKitAPI/TestWebKitAPI.xcodeproj/project.pbxproj:
* TestWebKitAPI/Tests/WTF/StringBuilder.cpp: Added.
(operator<<):
(TestWebKitAPI::expectBuilderContent):
(TestWebKitAPI::expectEmpty):
(TestWebKitAPI::TEST):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@95770 268f45cc-cd09-0410-ab3c-d52691b4dbfc