kickerplugin: Add an autotest for RunnerModel and fix some issues

This is a mostly blind patch for bug 414920 which I failed to reproduce, but when looking at the code I spotted some issues. The backtraces in the bugreport all point to countChanged() causing the crash. I noticed while looking at the model that it is emitted multiple times, once for each row changed. In addition, this can happen while rows are being changed, which leads to extra potential problems. I added an autotest with a QAbstractModelTester which additionally found a problem when the model appends new rows.