Add a test helper for testing `attr.Type` / `attr.Value` implementations

There are a few behaviors that `attr.Type` and `attr.Value` implementations must adhere to that aren't enforced through the compiler but can be trivially unit tested for. We should consider adding a helper function to run those tests as part of the `attr` package. See an example from a long-ago prototype: https://github.com/paddycarver/tf-framework-future/blob/main/type_testing.go