Add analyzer to flow TestContext.CancellationToken in tests

[Youssef1313]

[Evangelink]

Cases to cover:

• overloads with token not being called
• overloads with default or CancellationToken.None provided
• let's report even if TestContext property or field is not available

Let's have a codefix.

We could reuse the equivalent analyzer from dotnet/roslyn-analyzers repo.