Add unit tests for nginx runtime manager

[sjberman]

As a developer,
I want my code to be thoroughly unit tested,
so that I can ensure that everything works as expected.

The nginx runtime manager is lacking in unit test coverage.

Acceptance

• Add unit tests to thoroughly cover the nginx runtime manager code

Dev notes

• will likely need mocks for nginx plus client and "verifyClient"
• use ginkgo since we're testing interfaces

[miledxz]

Heey @sjberman, feel free to assign me, would be cool to add some tests there !

[mpstefan]

Hey @zedGGs, thanks for the help on this. Assigned the issue to you and will pull your PR into the sprint when we see it!

Let us know if you need any help!

[miledxz]

Heey @mpstefan, awesome stuff !

There's one thing, adding tests for interfaces, for example this one:
https://github.com/nginxinc/nginx-gateway-fabric/blob/main/internal/mode/static/nginx/runtime/manager.go#L49

that's actually being used in ManagerImpl, do you prefer using counterfeiter here ? Or to manually mock this interface,
I have checked few other examples, maybe this one: https://github.com/nginxinc/nginx-gateway-fabric/blob/main/internal/framework/events/first_eventbatch_preparer.go#L38,

and then in tests theres is a fakeReader, but still not sure does these examples correlate,

Thank you in advance

[sjberman]

@zedGGs Feel free to use counterfeiter to mock any interfaces that need it.