Install coverlet.msbuild for code coverage

Author: asbjornu

test/json-ld.net.tests/json-ld.net.tests.csproj

@@ -20,6 +20,7 @@
   </ItemGroup>
 
   <ItemGroup>
+    <PackageReference Include="coverlet.msbuild" Version="2.3.1" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.5.0" />
     <PackageReference Include="xunit" Version="2.3.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.3.1" />