[GTK] Make the 32 bits bot only build and run API tests
https://bugs.webkit.org/show_bug.cgi?id=113532

Reviewed by Martin Robinson.

* BuildSlaveSupport/build.webkit.org-config/config.json: make the 32 bits bot use the new BuildAndAPITest type
* BuildSlaveSupport/build.webkit.org-config/master.cfg:
(BuildAndAPITestFactory): new factory that builds and runs API tests, only used by the GTK+ 32 bits bot at the
moment;
(BuildAndAPITestFactory.__init__): add API tests step for platforms that have it.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148745 268f45cc-cd09-0410-ab3c-d52691b4dbfc