test: make IsolateData per-isolate in cctest

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.