strong-naming error needs explicit ref in tests

Author: mgravell

tests/protobuf-net.Grpc.Test/protobuf-net.Grpc.Test.csproj

@@ -20,6 +20,7 @@
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>
     <ProjectReference Include="..\..\src\protobuf-net.Grpc\protobuf-net.Grpc.csproj" />
+    <PackageReference Include="Grpc.Core.Api" Version="$(GrpcVersion)" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFramework)' != 'net461' and '$(TargetFramework)' != 'net472'">