Cluster-api-provider-aws: Unit testing for AWSMachine controller

[rudoi]

/kind feature

Describe the solution you'd like
[A clear and concise description of what you want to happen.]
We should have some more comprehensive unit testing around AWSMachine reconciliation, perhaps using mocks and stuff.

We should test AWSMachines as they move through phases.

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• Cluster-api-provider-aws version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):

[vincepri]

/priority important-soon
/milestone v0.4

[liztio]

/assign
/lifecycle active

[liztio]

/unassign
/remove-lifecycle active

This is a hairy problem. The machine controller has several hundred-line functions, takes in multiple giant blocks of mutable state, and has almost no modularity. I don't have any confidence in my ability to write tests that will actually get used.

[liztio]

here's my diagram of the requirements of just one reconcile function

[liztio]

/assign
/lifecycle active

past!liz was a coward, present!liz is forging onwards