The `Context::SlowGetAlignedPointerFromEmbedderData()` method returns
a pointer, so the fact that it allocates handles is not obvious to
the caller.

Since this is the slow path anyway, simply add a handle scope inside
of it.

The tests are also modified to perform the same check for the
`Object` equivalent of this method.

Change-Id: I5f03c9a7b70b3a17315609df021606a53c9feb2d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1879902Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64583}