Rework stack_controller_test.go to test mechanisms in isolation

[squaremo]

stack_controller_test.go has tests of specific mechanisms (e.g., using a config that comes in a file with the program) along with smoke tests that run an example AWS-based stack using various means of providing config, secrets, etc.

There are two things to improve here:

• don't rely on the AWS provider -- that doesn't really test anything other than that pulumi works as expected -- so the tests don't need AWS credentials, and complete faster
• separate out each mechanism into its own test, so that failures are easier to diagnose