Add missing coverlet.msbuild package for code coverage

Author: lassevk

tests/DiffLib.Tests/DiffLib.Tests.csproj

@@ -29,6 +29,10 @@
             <PrivateAssets>all</PrivateAssets>
             <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
         </PackageReference>
+        <PackageReference Include="coverlet.msbuild" Version="6.0.3">
+          <PrivateAssets>all</PrivateAssets>
+          <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+        </PackageReference>
         <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
         <PackageReference Include="NUnit" Version="4.3.2" />
         <PackageReference Include="NUnit.Analyzers" Version="4.5.0">