Bump test projects up to .NET 4.5.2

- aspnet/Testing#248
- xUnit no longer supports .NET 4.5.1

Author: dougbu

test/Microsoft.AspNetCore.JsonPatch.Test/Microsoft.AspNetCore.JsonPatch.Test.csproj

@@ -1,7 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <Import Project="..\..\build\common.props" />
   <PropertyGroup>
-    <TargetFrameworks>netcoreapp1.1;net451</TargetFrameworks>
+    <TargetFrameworks>netcoreapp1.1;net452</TargetFrameworks>
+    <TargetFrameworks Condition=" '$(OS)' != 'Windows_NT' ">netcoreapp1.1</TargetFrameworks>
   </PropertyGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\src\Microsoft.AspNetCore.JsonPatch\Microsoft.AspNetCore.JsonPatch.csproj" />
@@ -14,7 +15,7 @@
   <ItemGroup Condition=" '$(TargetFramework)' == 'netcoreapp1.1' ">
     <PackageReference Include="System.Diagnostics.TraceSource" Version="4.4.0-*" />
   </ItemGroup>
-  <ItemGroup Condition=" '$(TargetFramework)' == 'net451' ">
+  <ItemGroup Condition=" '$(TargetFramework)' == 'net452' ">
     <Reference Include="System.Linq.Expressions" />
     <Reference Include="System" />
     <Reference Include="Microsoft.CSharp" />