Make tests less noisy

[jordi-pivotal]

• Avoids printing stacktraces in tests that check failing rake tasks
• Accepts VERBOSE environment variable to print output when shelling out

[ragaskar]

A couple of us talked about this and decided we liked the verbosity. I guess there's a world where we just turn verbose on for CI and leave it off for development, but if the verbosity is useful for CI it's probably also useful for dev. Closing.