variables_panel_data_sets_ask_off_the_ui_thread checked only that the
exported file started with the header, so an export of the header
alone passed. Its folder was named after the process ID and was used
as it was found, so an out.csv left by an earlier run with the same ID
passed the check even when the export wrote nothing.
The test now empties its folder from vectorcraft_testkit::temp_dir
before use, and checks that the file holds the header and the row for
data set B and nothing else.