Introduce a platform xUnit trait to indicate platform specific tests like this

[Fact]
[Trait("Platform", "WindowsOnly")]
public void PushCommand_PushToFileSystemSourceUncStyle()
{
...
}

Having the trait one can invoke xunit runner with -ExcludeTraits "Platform=WindowsOnly"

Opened as a follow up on:
#1053
NuGet/NuGet.Client#253