Incorporate log formatter when solely using `XunitTestLogger`

[stijnmoreels]

Is your feature request related to a problem? Please describe.
When the XunitTestLogger is solely used with the translation between xUnit's ITestOutputHelper and Microsoft's ILogger, there's no place to register a formatter that let's you control how a state is being formatted.

Describe the solution you'd like
It would be nice if the XunitTestLogger would take into account the BeginScope so that log entries could be grouped together in the test output.

Additional context
Because we could be working outside a running host application, there is no application services or scopes available to 'hook' the logging scope on. Therefore, it will probably be a matter of saving/removing the state ourselves.

🚩 Also do not forget scenarios with multiple scopes, and how that would effect this logging scopes.