[GTK] Do not include JavaScriptCore stress tests in release tarballs
https://bugs.webkit.org/show_bug.cgi?id=135503

Reviewed by Martin Robinson.

* gtk/manifest.txt: Exclude Source/JavaScriptCore/tests dir.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@171986 268f45cc-cd09-0410-ab3c-d52691b4dbfc