test coverage for initial build, no change build, changed build

[miabbott]

As part of our CI, it would be useful to have a set of compose tests that exercises the different build "state" that one could find themselves in:

• initial build (nothing previous)
• no changes build (previous build, but nothing changed)
• changed build (previous build, changes detected)

This would help us catch corner cases that may crop up, like we saw in https://github.com/coreos/coreos-assembler/pull/1688

Might be a costly set of tests in terms of the amount of time required, so perhaps there is smaller set of unit tests that could cover these scenarios.