Tests: upgrade to .NET 6

This commit upgrades Test project target platform to .NET 6.
There's no excuse to use .netcoreapp 3.1 for our test project
anymore. For now we will keep using .NET Core 3.1 to build nuget
packages.

.github/workflows/CI.yml

@@ -10,10 +10,10 @@ jobs:
       - uses: actions/checkout@v2
         with:
           submodules: recursive
-      - name: Setup .NET Core SDK 3.1.x
+      - name: Setup .NET 6
         uses: actions/[email protected]
         with:
-          dotnet-version: '3.1.x'
+          dotnet-version: '6.0.x'
       - name: Install dependencies
         run: dotnet restore
       - name: Build

NOnion.Tests/NOnion.Tests.csproj

@@ -1,15 +1,14 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <TargetFramework>net6.0</TargetFramework>
 
     <IsPackable>false</IsPackable>
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="NUnit" Version="3.12.0" />
-    <PackageReference Include="NUnit3TestAdapter" Version="3.16.1" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.5.0" />
+    <PackageReference Include="NUnit" Version="3.13.3" />
+    <PackageReference Include="NUnit3TestAdapter" Version="4.5.0" />
     <PackageReference Include="Portable.BouncyCastle" Version="1.8.10" />
   </ItemGroup>