ScavengeExternalString and ScavengeExternalOneByteString clear GC flags,
which confuses concurrent sweeping that was started from another test.

Tests that mutate flags should not be threaded.

Bug: v8:7671
Change-Id: I08656d06fe85ff45baca685ebe5982528aad774c
Reviewed-on: https://chromium-review.googlesource.com/1019102Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52690}