* Fix GH-17658: COMPersistHelper::LoadFromStream() can segfault
The actual fix is trivial, but to be able to test the behavior we have
to introduce an own COM object, since existing persistable objects
likely implement `IPersistInit`, not only `IPersist`. We also want to
avoid further test dependencies on possibly unavailable objects, such
as `Word.Application`.
To this purposes, we add a small COM in-process server, which may be
extended for other testing purposes. We keep it simple by implementing
it in C++, but without using any more sophisticated frameworks like ATL.
This component needs to be built explicitly (`nmake comtest.dll`), and
also needs to be explicitly registered (`nmake register_comtest`).
When no longer needed, it is possible to unregister the component
(`nmake unregister_comtest`).
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