Move tests to _test package

[briggySmalls]

Apparently it is possible to put tests both alongside the source they test and in a different package (as long as it is suffixed 'test').

This is good because it prevents one from testin internals

[briggySmalls]

An interesting implication of this is that all the New methods would need to be used in the tests.

At the moment I have a pattern where lower-level AWS service clients are instantiated within a New call from the session which is passed in. The unit tests then, in order to inject mocks, don't use the New call, instead creating an internal struct instance.

Using a _test package would make this impossible, requiring AWS instances to be injected.

[briggySmalls]

It would probably be necessary to move mocks to a separate mocks directory/package. Which might be quite nice (easy to .gitignore and clean).

The impulse here would be to move mocks to a the test package too. But this doesn't work:

found packages email (email.go) and email_test (mock_ses.go) in /Users/sambriggs/Code/go/detectordag/shared/email

Note that it doesn't complain that email_test.go has the package email_test, just that mock_ses.go does. That's because it seems there is some check on the filename.