Add configuration entry to send DATA with a "TEST" flag.

[nigini]

As we are moving on into having no separate database for testing, it would be great to be able to only get a certain selection of the dataset that is (or not) part of testing the study. I imagine something like:

• in the study.js: add a param.test = true (to be set at build time)
• also in study.js: add a URL_PARAM that will set that param.test to true is present: ?LITW_TEST=true
• finally, in litw.data the _submit call used by all data calls, will add a testing: true to the data!