- circular buffer can return its count of elements (even though a better solution may be implemeted later)
- stub_motion can handle multiple planned moves
- improved load/unload filament tests
- fix homing procedure for Idler and Selector
(homing now ends with a move to the Parking position)
- fix unit tests' startup conditions with regard to necessary
homing of Idler and Selector
TODO: still test_cut_filament fails for minor reasons
Besides Unload Filament, which only operates on active slot, all other
top level state machines check the validity of the command's parameter.
If the parameter is out of range for available slots, they return
ErrorCode::INVALID_TOOL now.