testrunner has tests asserting about the implementation details, assertions
about the behavior of the testrunner (already in-place) provides robust coverage
already.

This cl remove the brittle assertions.

R=machenbach@chromium.org
CC=yangguo@chromium.org,sergiyb@chromium.org

Bug: v8:8174
Change-Id: I6583c971b7cf7eb2eb7dfa2b6737d6aa67957feb
Reviewed-on: https://chromium-review.googlesource.com/c/1421359
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Belozorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58928}