Review & fix prepare.spec

During the review of #121 I noticed that the tests in the `describe('module.exports.prepare method'` block in `tests/spec/unit/templates/cordova/lib/prepare.spec.js` were in pretty bad shape.

Funky stuff going on there:

- instance methods being mocked with arrow functions to return `this`
- massive amount of mock setup and expectations duplicated among the tests
- test bodies wrapped in asynchronous wrapper although nothing apparently asynchronous is going on there (or at leas it is not handled correctly)

Open tests/spec/unit/templates/cordova/lib/prepare.spec.js and inspect the describe('module.exports.prepare method' block first. Review the existing mocks, expectations, and async wrappers, then run the focused prepare.spec tests; done means the duplicated setup is reduced, instance methods are mocked appropriately, unnecessary async handling is removed, and the tests still pass.

Written by the indexing model from the issue text.