Most V8 unit tests are multi-threaded, so configure GTest to use the
thread-safe GTest implementation by default.

This can be overridden on a per-test basis by setting:
  testing::FLAGS_gtest_death_test_style = "fast";
during the test's SetUp() (see the GTest documentation for details).

Bug: v8:10143
Change-Id: I7414c5d8ae22eb8d9b8c4813f958ca571e1d0310
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2011085
Commit-Queue: Wez <wez@chromium.org>
Auto-Submit: Wez <wez@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65873}