run-api-tests should have an option to specify root
https://bugs.webkit.org/show_bug.cgi?id=108210

Reviewed by Ryosuke Niwa.

* Scripts/run-api-tests:
Add the option and use it to set the configuration product directory. Since it is supposed
to point to the built products, do not build the products if root is specified (this mimics
the behavior of run-javascriptcore-tests).

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141172 268f45cc-cd09-0410-ab3c-d52691b4dbfc