[test][cleanup] Clean up value-serializer-unittest.cc

Refactor the helpers to use return values instead of continuation functors.
This reduces compilation time in Release mode from 30s to 10s (Debug: 10->7s).
Also shorten the boilerplate code a bit.

Bug: v8:7310
Change-Id: Icf6309e4fd5478a1f55979112d1219bc7eaf4e6d
Reviewed-on: https://chromium-review.googlesource.com/938316
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51584}