Further extend the argument hint code completion test coverage

Summary:
Also check whether we find matching declarations, and add test
code that uses namespaces. This uncovers a deficit in libclang,
namely clang_getCompletionParent on the overload candidate completion
strings does not return any valid string. Without a valid parent
we cannot find the namespaced declarations and thus fail there...

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D9744