Update DiffEngine.Tests.csproj

Author: SimonCropp

src/DiffEngine.Tests/DiffEngine.Tests.csproj

@@ -8,7 +8,7 @@
   </PropertyGroup>
   <ItemGroup>
     <Using Include="DiffEngine" />
-    <PackageReference Include="MarkdownSnippets.MsBuild" Version="25.0.1" />
+    <PackageReference Include="MarkdownSnippets.MsBuild" Version="25.0.1" PrivateAssets="all" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.7.0" />
     <PackageReference Include="Argon" Version="0.7.2" />
     <PackageReference Include="Xunit" Version="2.5.0" />