Add unit test for subscription

[gojanpaolo]

I'm currently trying to figure out how to add a unit test for subscription. :)

References:
https://github.com/ChilliCream/graphql-workshop/blob/master/docs/8-testing-the-graphql-server.md
https://chillicream.com/docs/hotchocolate/api-reference/aspnetcore#subscription-session-handling

[gojanpaolo]

I still can't figure it out. 😢 Will work on something else in the meantime.

[gojanpaolo]

https://github.com/ChilliCream/hotchocolate/blob/10.4.2/src/Core/Types.Tests/Types/SubscriptionTypeTests.cs#L359