[Epic]: Remove FluentAssertions from TestKit

[Aaronontheweb]

FluentAssertion's commercial license change is a problem for us. Now as an OSS project, we don't technically have to pay for a license on the commercial versions (8+) - which is fine. But, our customers who use the TestKit now consume a FluentAssertions dependency transitively due to our decision we made in the v1.5 release pipeline here: #5432

I'm not cool with passing that risk onto end-users, so I think we should remove all FluentAssertions usage from the TestKits (AND we should probably make some steps to remove our usage of it from the project in general, but this is like a 1000000x lower priority than removing it from the TestKit.)

TestKits

Remove FA from all base class methods and remove it as a dependency.

• Akka.TestKit.Nunit (does not contain reference to FA)
• Akka.Hosting.TestKit
• Akka.Serialization.TestKit

Tests

The tests are a much bigger issue due to sheer surface area involved and honestly, it's probably not a high priority, but we want to slam the gate shut on someone reintroducing it or anything like it back into the TestKit.

I've used this for all of the our xUnit-based tests in some other projects before and it worked pretty well https://github.com/amoerie/fluentassertions-migrator - not 100%, but let's say 80-90%

[Aaronontheweb]

Worth noting: the versions of FA we use right now are all FOSS, so there's no viral commercial license transmission to customers, but all it takes is someone running dependabot or a NuGet update script to get hit with that in their own projects.

[cbersch]

Have you considered using AwesomeAssertions?

[Aaronontheweb]

@cbersch we're not taking any more third party dependencies for something trivial like assertions. We can probably get by just using xUnit's built-in assertions.