Better organize the new Bounty#destroy() tests.
Don't import SessionsHelper into the Bounty model tests.

This breaks the new Bounty model tests, but they aren't actually model
tests--they're testing the Bounty controller's destroy() method and are
meant to be placed in the Bounty controller tests, not the Bounty model
tests. They will be moved in a future commit.
Add a stub for a test to check that Bounty.destroy() does not work on accepted bounties.
Remove a stray tab.