Implement unit test with in-memory server implementation.

[benbjohnson]

Change Summary

This pull request changes the mock implementation of the deploy test to use a simple, in-memory server implementation. This should help us to build tests faster and more consistently by not having to mock every since call.

Related to: https://github.com/superfly/fly-go/pull/69

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Documentation

• [ ] Fresh Produce
• [ ] In superfly/docs, or asked for help from docs team
• [x] n/a