Use the same version of Xunit as in corefx (2.4.1 prerelease) (#20506)

Use the same version of Xunit as in corefx (2.4.1 prerelease) (#20506)

From https://xunit.github.io/releases/2.4.1-pre-4059

**xUnit.net 2.4.1 Pre-Release (build 4059)**

Core framework
* BUG: Fixed an issue with xunit.abstractions not correctly installing with older versions of Visual Studio (pre-2017).
* BUG: Fixed an issue with ByRef types with .NET Core 2.1.
* BUG: Removed an unnecessary first chance exception related to attempting to load configuration files.
* BUG: Fixed an issue where synchronous and asynchronous disposal of test classes were running in parallel. The documented behavior is that asynchronous disposal (via IAsyncLifetime) runs to completion before synchronous disposal (via IDisposable).

Console runner
* BUG: Fixed an issue with XML element order with the NUnit report.

MSBuild runner
* BUG: Fixed an issue with XML element order with the NUnit report.