Create "xunit" logger provider

[analogrelay]

In xunit, it is recommended that "console" output be written to ITestOutputHelper rather than the Console. We should provide a Logger to plug in to ILoggerFactory which writes to this. See https://github.com/aspnet/MusicStore/blob/dev/test/E2ETests/Common/XunitLogger.cs for an example.

/cc @natemcmaster

[Eilon]

We can certainly do this, but what would actually wire it up in our tests? And... what problem in our tests does it solve?

[analogrelay]

1. The new test host in dotnet test suppresses Console output. So if we want to write something out, we need to use this.

2. The ITestOutputHelper collects output for a single test and ensures it is rendered as a discrete unit, which is important when doing parallel testing

3. Tests would have to wire it up themselves. This is for tests that currently create a Logger Factory and attach the Console logger. Mostly functional tests that do deployment.

[Eilon]

Ah, got it, this makes sense. Are there any tests where we know we'd want to use this ASAP? I want to make sure that whoever I assign this to will have an actual use case for it 😄

[analogrelay]

Well there are places we already have it, https://github.com/aspnet/SignalR/blob/dev/test/Microsoft.AspNetCore.SignalR.Tests/XUnitLoggerProvider.cs , https://github.com/aspnet/MusicStore/blob/dev/test/E2ETests/Common/XunitLogger.cs , etc.

So this more about making it a common component, to avoid copy 🍝

[analogrelay]

Closing in lieu of aspnet/Logging#574