migrate to NUnit 4.x

Author: smdn

tests/Directory.Build.props

@@ -26,9 +26,16 @@ SPDX-License-Identifier: MIT
   />
 
   <ItemGroup Condition=" '$(IsTestProject)' == 'true' ">
-    <PackageReference Include="NUnit" Version="3.13.3" />
-    <PackageReference Include="NUnit3TestAdapter" Version="4.4.2" />
+    <PackageReference Include="NUnit" Version="4.0.1" />
+    <PackageReference Include="NUnit3TestAdapter" Version="4.5.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0"/>
+    <!--
+      Workaround for the NUnit issue.
+      Manually add a reference to System.Threading.Tasks.Extensions as essentially required by NUnit.
+      This may be fixed in NUnit 4.1.
+      ref: https://github.com/nunit/nunit/issues/4581
+    -->
+    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.4" Condition="$(TargetFramework.StartsWith('net4'))" />
     <PackageReference
       Include="Smdn.MSBuild.DefineConstants.NETSdkApi"
       Version="[1.4.0]"