SignalDumper: with BlockingQueued, the indentation level for the slot should be +1 from signal level

With SignalDumper testlib prints out the signals and slots as they are being called (at least mostly, see QTBUG-74099) with increasing indentation as the signals and slots leads to emitting more signals.
In cross-thread scenarios they don't affect eachother because they are not tied together, but with a BlockingQueued scenario is would make sense to have the slot be indented according to the signal, even though it's in another thread.