Fix for issue #1065 NUnit to destroy ActorSystem after each test run inline with XUnit

[garethbudden]

I've made changes as outlined in #1065 to allow NUnit's TestKit to work in the same way as xunit.

Problems

Providing an ActorSystem in the constructor will fail

Unlike xunit (and I think mstest), NUnit does not create a new instance of the test class for each test run. If the user provides an ActorSystem in the constructor, all tests will attempt to use the same system, however after each test the system is destroyed. One possible fix would be to take a Func<ActorSystem> instead, however that would introduce a breaking change to the API for all TestKit implementations and I wasn't sure how you'd feel about that. Another option is simply to throw a NotSupportedException in the NUnit TestKit if the user provides an instance of ActorSystem.

Redundant methods

The NUnit TestKit has AfterAll(), Dispose(), and Dispose(bool disposing) methods, however these are redundant as the class now uses a TearDown attribute to clean up. Removing these methods makes sense but again introduces a breaking change. Would this be a problem?

[Aaronontheweb]

Another option is simply to throw a NotSupportedException in the NUnit TestKit if the user provides an instance of ActorSystem.

Code-bombing people is a much better alternative than introducing breaking changes to the API, so I approve :)

[Aaronontheweb]

The NUnit TestKit has AfterAll(), Dispose(), and Dispose(bool disposing) methods, however these are redundant as the class now uses a TearDown attribute to clean up. Removing these methods makes sense but again introduces a breaking change. Would this be a problem?

The TestKit API is stable as of 1.0. There's no possibility of changing it. NUnit must work with it.

[Aaronontheweb]

@garethbudden this looks great - just finished my review.

Could you have AfterAll / Dispose etc called inside the testfixture's TearDown-decorated methods? That should work.

[garethbudden]

I've added the code 💣, and made the TearDown method call AfterAll().

I haven't made the TearDown method call Dispose() however, because I realised this would mean that the test fixture instance would be Disposed multiple times, one for each test! Which felt wrong, and may cause adverse affects on people who rely on dispose being called once when the Fixture finishes.

[Aaronontheweb]

Ty sir!

[mchandschuh]

@Aaronontheweb -- nunit 3.13 adds [FixtureLifeCycle(LifeCycle.InstancePerTestCase)] which creates a new test class instance per test, but the not supported exception prevents it's use. What needs to be updated to allow this to work?

https://docs.nunit.org/articles/nunit/writing-tests/attributes/fixturelifecycle.html

[Aaronontheweb]

@mchandschuh we might need to patch Akka.NET first in order to fix this.