Explain what the aspire testing framework is for and how it works

[davidfowl]

There's lots of confusion as to what the aspire testing support provides. We need to explain what aspire testing is for and when to use it.

dotnet/aspire#7877

Aspire’s testing capabilities are for black box testing your entire distributed application, not for writing unit tests or doing white box integration testing. If you want to run your single project in memory and stub out the database then continue to use web application factory.

If you want to use a real database and talk to your API to make sure data is persisted into the real Postgres database then aspire testing is the way to go.