Add unit test for the calculation of "name" groups with expanded items

This prevents a possible regression that would have happened with the
first version of https://git.reviewboard.kde.org/r/112725/

The problem was that isChildItem(int index) would return "false"
incorrectly when the QHash for that item was not initialized yet. The
grouping code would then try to read the "text" from the empty QHash,
which yielded an empty QString, and then accessing the first character
of that string caused a crash.