Allow a test to include an optional Google Analytics slot number. If using ga.js, this would allow you to store the index of a custom variable (see https://developers.google.com/analytics/devguides/collection/gajs/gaTrackingCustomVariables). If using analytics.js, this would let you do the same thing with a custom dimension or metric (see https://developers.google.com/analytics/devguides/collection/analyticsjs/custom-dims-mets)