Add missing test package

Author: kzu

src/System.Diagnostics.Tracer.Tests/System.Diagnostics.Tracer.Tests.csproj

@@ -7,6 +7,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.4.1" />
+    <PackageReference Include="Microsoft.TestPlatform.ObjectModel" Version="17.4.1" />
     <PackageReference Include="Moq" Version="4.18.4" />
     <PackageReference Include="xunit" Version="2.4.2" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5" />