workaround for https://github.com/xunit/xunit/issues/1573

Rx.NET/Source/tests/Tests.System.Reactive/Tests.System.Reactive.csproj

@@ -2,6 +2,7 @@
   <PropertyGroup>
     <TargetFrameworks>net46;netcoreapp1.1;netcoreapp2.0</TargetFrameworks>
     <NoWarn>$(NoWarn);CS0618</NoWarn>
+    <RuntimeFrameworkVersion Condition=" '$(TargetFramework)' == 'netcoreapp2.0' ">2.0.0</RuntimeFrameworkVersion>
   </PropertyGroup>
   
   <ItemGroup>