To do this, you need to adjust the Test Settings, available in the Test menu. If you leave the default settings, and you are using csv input, the test will only run once (i.e. take the first line from csv file).
Within dialog, select Web test then “One run per data source row”
It is recommended to use “Save As …” option, so you keep the original setting intact. Then to switch the test settings as/when needed.