add System.Collections.Immutable

Author: mgravell

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

@@ -12,6 +12,7 @@
 	<ItemGroup>
 		<PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.6.1" />
 		<PackageReference Include="protobuf-net" Version="3.0.0-alpha.154" />
+		<PackageReference Include="System.Collections.Immutable" Version="1.7.1" />
 		<PackageReference Include="xunit" Version="2.4.1" />
 		<PackageReference Include="xunit.runner.visualstudio" Version="2.4.2">
 			<PrivateAssets>all</PrivateAssets>