Revert NUnit to 3.13.3 - no upgrade needed for MTP

The NUnit 4.x upgrade broke all host-side test projects because NUnit 4
removed classic assert APIs (Assert.IsTrue, FileAssert, etc.).

Only NUnit3TestAdapter 5.0.0 is needed (for AddNUnit() MTP support) in
the device test projects. NUnit itself stays at 3.13.3, and the shared
NUnitReferences.projitems is restored to its original versions.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jonathanpeppers

build-tools/scripts/NUnitReferences.projitems

@@ -1,9 +1,9 @@
 <Project>
   <!-- This file assumes Configuration.props has been imported -->
   <ItemGroup>
-    <PackageReference Include="NUnit"               Version="4.3.2" />
+    <PackageReference Include="NUnit"               Version="3.13.3" />
     <PackageReference Include="NUnit.ConsoleRunner" Version="$(NUnitConsoleVersion)" />
-    <PackageReference Include="NUnit3TestAdapter"   Version="5.0.0" />
+    <PackageReference Include="NUnit3TestAdapter"   Version="4.4.2" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.5.0" />
   </ItemGroup>
 </Project>

tests/CodeGen-Binding/Xamarin.Android.JcwGen-Tests/Xamarin.Android.JcwGen-Tests.csproj

@@ -35,7 +35,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="NUnit" Version="4.3.2" />
+    <PackageReference Include="NUnit" Version="3.13.3" />
     <PackageReference Include="NUnit3TestAdapter" Version="5.0.0" />
   </ItemGroup>
 

tests/Mono.Android-Tests/Mono.Android-Tests/Mono.Android.NET-Tests.csproj

@@ -65,7 +65,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="NUnit" Version="4.3.2" />
+    <PackageReference Include="NUnit" Version="3.13.3" />
     <PackageReference Include="NUnit3TestAdapter" Version="5.0.0" />
   </ItemGroup>
 

tests/TestRunner.Core/TestRunner.Core.NET.csproj

@@ -10,7 +10,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="NUnit" Version="4.3.2" />
+    <PackageReference Include="NUnit" Version="3.13.3" />
     <PackageReference Include="NUnit3TestAdapter" Version="5.0.0" />
     <PackageReference Include="Microsoft.Testing.Extensions.TrxReport" Version="$(MicrosoftTestingPlatformVersion)" />
   </ItemGroup>