Downgrade NUnit and NUnit3TestAdapter

Author: Paul Johnson

src/Umbraco.Tests/Umbraco.Tests.csproj

@@ -104,8 +104,8 @@
     <PackageReference Include="MiniProfiler" Version="4.0.138" />
     <PackageReference Include="Moq" Version="4.16.1" />
     <PackageReference Include="NPoco" Version="3.9.4" />
-    <PackageReference Include="NUnit" Version="3.13.2" />
-    <PackageReference Include="NUnit3TestAdapter" Version="4.0.0" />
+    <PackageReference Include="NUnit" Version="3.11.0" />
+    <PackageReference Include="NUnit3TestAdapter" Version="3.12.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.5.0" />
     <PackageReference Include="Newtonsoft.Json" Version="12.0.1" />
     <PackageReference Include="Owin" Version="1.0" />