test csproj, add missing PackageReference to "Newtonsoft.Json"

Author: Peter Metz

test/Serilog.Formatting.Compact.Tests/Serilog.Formatting.Compact.Tests.csproj

@@ -13,6 +13,7 @@
 
     <ItemGroup>
         <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.6.3" />
+        <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
         <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5">
             <PrivateAssets>all</PrivateAssets>
             <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>