[iOS] Don't build DumpRenderTree, gtest, or TestWebKitAPI

[iOS] Don't build DumpRenderTree, gtest, or TestWebKitAPI
https://bugs.webkit.org/show_bug.cgi?id=127364

Reviewed by Simon Fraser.

For now, don't build DumpRenderTree and TestWebKitAPI for iOS as these
projects fail to build because we need to upstream the relevant iOS changes.
Also, don't build gtest, which is a dependency of TestWebKitAPI since we
aren't building TestWebKitAPI.

* Scripts/build-webkit:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@162471 268f45cc-cd09-0410-ab3c-d52691b4dbfc