Replace `Microsoft.AspNetCore.Analyzer.Testing` with `Microsoft.CodeAnalysis.Testing`

Currently, our analyzers/codefixes rely on a custom library to validate their behavior. Moving forward, we can use the official testing libraries provided in the `M.CA.Testing` and move away from relying on our custom setup.

At minimum, new analyzers should use the M.CA.Testing library for validation. We can also invest some time in porting over the older tests and removing the custom project all together.