cmd/jujud: added supporting logic to have state servers sync before starting upgrades

This change has no functional impact but implements the machine agent changes to make state machine agents wait for each other to be ready before proceeding with an upgrade. Right now no wait is actually performed. TODOs have been inserted for the parts that rely on upcoming changes.

Note to reviewers: this PR includes 2 commits the first of which is a rearrangement of the machine agent upgrade tests to aid clarity. It may be easier to follow the changes in this PR if the commits are viewed separately.