Add testing support for .NET Framework (#113)

* Add testing support for .NET Framework

The StringExtensions class is to work around the combination of multiple calls to these specific methods with those methods not being supported in .NET Framework and the linting requiring these calls in .NET Core.

* .NET 4.7.2 targeting pack for non-Windows systems

* Add missing framework path overrides based on xUnit docs

* Include System.Runtime reference

* Added System.Threading.Tasks reference

* Reverting the more complicated .NET Framework support config

* Potential fix for cross-platform testing

For exception: Could not load file or assembly 'Microsoft.VisualStudio.TestPlatform.ObjectModel, Version=11.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'

Author: Turnerj

Tests/Schema.NET.Test/Schema.NET.Test.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup Label="Build">
-    <TargetFrameworks>netcoreapp3.1</TargetFrameworks>
+    <TargetFrameworks>net472;netcoreapp3.1</TargetFrameworks>
     <CodeAnalysisRuleSet>../../MinimumRecommendedRulesWithStyleCop.ruleset</CodeAnalysisRuleSet>
     <LangVersion>latest</LangVersion>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
@@ -12,7 +12,10 @@
   </ItemGroup>
 
   <ItemGroup Label="Package References">
+    <!-- See https://github.com/Microsoft/dotnet/tree/master/releases/reference-assemblies -->
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" PrivateAssets="All" Version="1.0.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.4.0" />
+    <PackageReference Include="Microsoft.TestPlatform.ObjectModel" Version="16.4.0" />
     <PackageReference Include="Moq" Version="4.13.1" />
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.1" />

Tests/Schema.NET.Test/StringExtensions.cs

@@ -0,0 +1,19 @@
+#if NET472
+namespace Schema.NET.Test
+{
+    using System;
+
+#pragma warning disable IDE0060 // Remove unused parameter
+#pragma warning disable CA1801 // Remove unused parameter
+
+    public static class StringExtensions
+    {
+        public static string Replace(this string target, string oldValue, string newValue, StringComparison stringComparison) => target.Replace(oldValue, newValue);
+
+        public static int GetHashCode(this string target, StringComparison stringComparison) => target.GetHashCode();
+    }
+
+#pragma warning restore CA1801 // Remove unused parameter
+#pragma warning restore IDE0060 // Remove unused parameter
+}
+#endif