Support xunit v3

[martincostello]

Is your feature request related to a problem? Please describe.

Add support for xunit v3: What's New in v3

Describe the solution you'd like

The ability to log to the relevant primitives when targeting xunit v3.

Migrating from v2 to v3

[teneko]

Thanks for migrating to v3 in the near future.

During that migration here a quick work around to get the console output:

• migrate to xUnit v3 by following What's New in v3
• use [assembly: CaptureTrace] to capture Debug.Write and Debug.Trace
• and instruct the logger factory to output to Debug (not Console), for example:

LoggerFactory.Create(static x => {
    x.AddDebug();
    // ...
})

and use that logger factory in a class fixture or test classes.