Update dependency NUnit to v4

Author: renovate[bot]

IDLImporterTests/IDLImporterTests.csproj

@@ -9,7 +9,7 @@
 
   <ItemGroup>
     <PackageReference Include="GitVersion.MsBuild" Version="5.12.0" PrivateAssets="All" />
-    <PackageReference Include="NUnit" Version="3.14.0" />
+    <PackageReference Include="NUnit" Version="4.0.1" />
     <PackageReference Include="SIL.ReleaseTasks" Version="2.5.0" PrivateAssets="All" />
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.1.1" PrivateAssets="All" />
   </ItemGroup>