... when addBinding is called with contextId. Previously, due to
a subtle type, we exposed bidings added with executionContextId to
all contexts created after the binding was added.

Also, do not persist context-specific bindings to agent state,
as context ids don't make sense across the process.

This also adds a test instrastructure to create additional context in
given context group.

Change-Id: I1b3e96cb65b756424bc7872d200bbbf41e4c30b8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2440982Reviewed-by: Simon Zünd <szuend@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70261}