retrace: Add --show-test-stdout arg to validation This argument is checked when running validation, so we have to define it. Bug: angleproject:7800 Change-Id: Ia24a8c8a82a8bdcefcf707bd9c9183ead4ee095d Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3990622 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Gert Wollny <gert.wollny@collabora.com>