[JUJU-1454] Re-write application_unit_test.go to use gomock generated mocks

[jack-w-shaw]

This is in part to build a foundation to convert the JujuConnSuite test in this package

Checklist

• [x] Code style: imports ordered, good names, simple structure, etc
• [x] Comments saying why design decisions were made
• [x] Go unit tests, with comments saying what you're testing
• ~[ ] Integration tests, with comments saying what you're testing~
• [ ] doc.go added or updated in changed packages

QA steps

Running the test suite should be enough

go test github.com/juju/juju/apiserver/facades/client/application

[benhoyt]

Thanks for filling out the checklist. Could you please add a basic doc.go for the apiserver/facades/client/application package while you're in there?

[jack-w-shaw]

@hmlanigan I believe I have addressed your final concerns, ready for another glance

And yes, I intend to squash before merge

[basebandit]

LGTM perhaps @hmlanigan can give another glance.

[jack-w-shaw]

/merge