Register vk.Stop() when spawning VKs in tests

[rflandau]

Currently, spawnVK(), a helper function for testing the vaultkeeper package, spawns a vk but does not register vk.Stop() to the tests's clean up functionality. As .Stop() is idempotent and good practice is to ensure all VKs are stopped during cleanup, spawnVK should just register vk.Stop() prior to returning.

The fix is simple enough: call t.Cleanup(vk.Stop) inside of spawnVK(). PRs resolving this issue should be sure to remove the now superfluous, external t.Cleanup() and defer calls.