Build TestUtilities library for netstandard, not netcoreapp.

src/Json.Schema.TestUtilities/Json.Schema.TestUtilities.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>netcoreapp2.0</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0</TargetFrameworks>
     <IsPackable>false</IsPackable>
   </PropertyGroup>