Optimize bakery's testing

[ianpittwood]

We recently had to push the test job to using ubuntu-latest-4x runners after adding tests that involved building a multidimensional matrix. Tests are now pushing the 20min mark so it may be time to consider going back and streamlining/optimizing our testing to ensure testing feedback is quick and actionable.

Some options to consider:

• Avoid invoking Docker and builds where possible and mocking those interactions for unit tests.
• Split "end-to-end" or "integration" type tests into a separate job so feedback from unit tests can be quicker while still allowing for longer tests to act.
• Use more "no-op" type Containerfile definitions if we are building artifacts where possible rather than truly building out images.
• Ensure that any images we do have to build are cleaned up immediately during teardown of the test that invoked them.