[unified-heap] Update unit tests for unified heap

Use GC without scanning stack for testing purposes when necessary.

A different strategy for the current architecture would be making tests
truly async and scheduling GC from the event loop.

Bug: 843903
Change-Id: Ib479d4a090bc04cbc4fe28522f0b9e084f34181d
Reviewed-on: https://chromium-review.googlesource.com/c/1292878
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601505}
2 files changed