add unit test for crash 313769

enable dynamic word wrap

1. open kwrite, resize window to 900x1000 pixel
2. open test file "data/bug313769.cpp"
3. <cursor down><cursor down><ctrl+shift+end><ctrl+shift+cursor-up>
4. <ctrl+pos1>
5. <ctrl+shift+minus>
6. undo, redo, undo

CCBUG: 313769