Completion rate increases for experiments users aren't participating in

I wrote this failing test https://github.com/philnash/split/commit/32f797198400392fda89fb1db91405338899fadf that shows the issue.

I'm not sure of the best way to fix this right now though, as the implementation of `doing_other_tests?` is not specific enough to calculate which test is being done. Do we need to save different data in the session so that we know what experiment is currently active?