Refactor common test code into code-coverage-utils.js and add tests to
verify counter behavior in opt/no-opt situations.

Bug: v8:6000
Change-Id: I07e62345476e8c81521c491ae605ddaf71600667
Reviewed-on: https://chromium-review.googlesource.com/584449Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46888}