This ensures that we only create one IsolateData for each isolate
inthe cctest, since IsolateData are meant to be per-isolate.
We need to make the isolate and isolate_data static in the
test fixtures as a result, similar to how the event loops and
array buffer allocators are managed in the
NodeZeroIsolateTestFixture but it is fine because gtest ensures
that the Setup() and TearDown() of the fixtures are always run
in order and would never overlap in one process.
PR-URL: https://github.com/nodejs/node/pull/48450
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
