fdo#75259: Add unit test for sharing of string pools with undo docs.

Change-Id: I785345144f7dd733b2ce8970412f79b384b8c94d
fdo#75259: Let undo document share its string pool with the source doc.

We already do that for the pool helper.  It would only make sense to do
it for shared strings as well.

Change-Id: I813d262799af6f0c1d3fa12246ad973f852ac199