tabbing_back_to_the_same_field_then_saving_keeps_the_committed_value (its
same-frame case) and typing_and_saving_in_the_same_frame_saves_the_last_keystrokes
both asked scratch() for "same-frame". The tests of the it binary share one
process id and run in parallel, so they got the same folder, and one could
clear it while the other was saving into it: the save then failed to read
back with NotFound. scratch() now adds a number taken per call.