The old tasks API is deprecated and we want to remove it in 8.0.
Thus, this CL refactors the platform tests to use the new
TaskRunner-based API, and removes redundant tests.

R=ahaas@chromium.org

Bug: v8:9810
Change-Id: Ie53c90184639e77b3247228059fd88290b233e0c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1868619Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64412}