Test processors can be enabled with --infra-staging flag.

Rerunning tests, execution and verbose progress indicator already
work as test processors.

Bug: v8:6917
Change-Id: I40fc42db94dbc8629e8000a3d363030045532fe3
Reviewed-on: https://chromium-review.googlesource.com/850398
Commit-Queue: Michał Majewski <majeski@google.com>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50399}