DUChain tests and PersistentMovingRange::text

[David Nolden]

The solution would be to finish PersistentMovingRange to also work
with not opened documents (using CodeRepresentation internally, as you
proposed).

The test-code could/should be inserted through a fake-url (this is
already implemented and working well, just look at the uses of
InsertIntoDUChain in test_cppcodegen.cpp), and then it would also work
with PersistentMovingRange.

You would simply have to use the more modern testing infrastructure as
in test_cppcodegen.cpp.

Greetings, David