Rework testing fixtures

[grepory]

I want us to move to an Interface + Mock methodology for unit testing. So instead of having an in-memory fixture store that's incapable of easily simulating all interactions with a Store (i.e. simulating error cases is brittle and/or non-trivial), I'd like us to move back to the mock store that was there before + testify.mock behavior simulation in faked methods per test. (We can develop some reusable faked methods if this ends up being super copypasta).

The key being that the mock should simulate behavio

[grepory]

The current fixtures package should go away, and knowledge of how to create fixtures for given models (structs) should reside next to the structs themselves within the test code for that package. E.g. in the types package, in check_test.go, we would have func FixtureCheck(string) *Check -- so that test code in another package can import types and call types.FixtureCheck("my_check") to get a generated check with an identifiable Name field.

[grepory]

@portertech @palourde @amdprophet -- I would love some feedback on this approach. If you'd like, I can concretize this in a PR this week so you can see exactly what I'm talking about.

[portertech]

Toss it into types! 👍

[portertech]

We now have https://github.com/sensu/sensu-go/blob/master/testing/mockstore/mock.go

[portertech]

We need to migrate older test code to use this method of fixtures.

[grepory]

Fixed with #97