Update Turkey.Tests to .NET 6

Otherwise we get an error since the main project is already targeting
.NET 6:

    dotnet-bunny/Turkey.Tests/Turkey.Tests.csproj : error NU1201:
    Project Turkey is not compatible with netcoreapp3.1
    (.NETCoreApp,Version=v3.1). Project Turkey supports: net6.0
    (.NETCoreApp,Version=v6.0)

Turkey.Tests/Turkey.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <TargetFramework>net6.0</TargetFramework>
 
     <IsPackable>false</IsPackable>
   </PropertyGroup>