[cleanup] Refactor codegen-tester

The implementation can be greatly simplified by using variadic
templates.

R=mstarzinger@chromium.org

Change-Id: Ifbda09bc536c9660a83d1888b395e92367c9b03e
Reviewed-on: https://chromium-review.googlesource.com/860458Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50603}