The inspector fuzzer is running into timeouts most of the time
currently, because the test explicitly needs to quit execution.
Make fuzzing more efficient by adding a watchdog thread which stop
execution after 2 seconds. This will still result in valid test cases,
i.e. everything that was executed within those two seconds will count as
covered code.

Drive-by: Slightly simplify the storage of task runners. No need to
clear the vector after termination.

R=szuend@chromium.org

Bug: chromium:1142437, chromium:1145285
Change-Id: I7b5fe7ddcbce731fbc3d74ee8c43f7249f34b918
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2520906
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71002}