The new helper function allows us to write tests for log parsing
without the need of first generating a log file.
This makes it easier guard against errors when the log format changes.

- add d8.log.getAndStop helper
- add basic log test
- fix test that regresses due to changed gc timing

Bug: v8:10668
Change-Id: Ie57171fa98fe90428b89c26289d55fcbf2a70615
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2403245Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Auto-Submit: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69987}