dEQP: Move libtester sources to a gyp variable. This keeps the target definition cleaner and will enable GN, if we ever get to that point. BUG=angleproject:998 Change-Id: I8692c93155a6bada9b1b0c05a1dd34af64a3b400 Reviewed-on: https://chromium-review.googlesource.com/280992 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org>