added common Fact and Theory attributes to Microsoft.DotNet.XUnitExtensions

[vlada-shubina]

Background: now with xunit/xunit#2073 it is possible to skip unit tests programmatically and this way works in VS, CI, etc.

There are common attributes that repos write usually based on platform and/or target framework, sometimes even more the once.
Microsoft.DotNet.XUnitExtensions looks like the common extensions provide by arcade and will be a better place for it, so that all repos that use arcade can use it too.

Repos that can benefit from it:
https://github.com/dotnet/sdk/blob/a30e465a2e2ea4e2550f319a2dc088daaafe5649/src/Tests/Microsoft.NET.TestFramework/Attributes/WindowsOnlyFactAttribute.cs
https://github.com/dotnet/installer/blob/8c476b58df6032c6713c69535d2679632abac9ba/test/Microsoft.DotNet.Tools.Tests.Utilities/WindowsOnlyFactAttribute.cs
https://github.com/dotnet/command-line-api/blob/72e86ec7615c0c8ecb6a13e34a5c0a97e9309909/src/System.CommandLine.Tests/Utility/WindowsOnlyFactAttribute.cs

• now dotnet/msbuild too: Replacing traits used to skip certain test cases with custom Theory and Fact attributes msbuild#8348