While we already had "one test to rule all stats", testing each method
individually makes reading, adding and changing tests easier.
Note we need to make all methods being tested public. We could also test
them using methods like `stats.generate[:total_valid_votes]` instead of
`stats.total_valid_votes`, but then the tests would be more difficult to
read.
Adding the option to assign a poll to a poll recount factory meant we
didn't need to create so much data.
Also note we're removing the `create(:poll_voter, origin: "booth")`
code, since it isn't used in the stats calculations.
