Progress indicator calculates the percentage using the estimated amount of
tests.

When base tests produce more tests or when testrunner filters some tests, the
percentage terminates over 100% or under it.

This CL adds an informative message about how the percentage behaves.

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

Bug: v8:8728
Change-Id: I91cafd2579ea1894ac347ff7483c307cd46c545d
Reviewed-on: https://chromium-review.googlesource.com/c/1477056Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59660}