Create a ServiceCollection,
var services = new ServiceCollection();
call your registration function and then assert that your restServiceType was added.
Next build a provider from the service collection, resolve the restServiceType
var provider = services.BuildServiceProvider();
var restService = provider.GetRequiredService(restServiceType);
and assert that it is created as desired.
The GetRequiredService extension method will throw an exception if the service is unable to resolve the target type.
Now that is based solely on what is currently being shown in your example as I am unaware of any other dependencies.