Remove internal-only methods from testing package

As mentioned in comments in #3099, there are some issues and limitation to the current unit test tools:

- It's hard to test a user-defined generic interceptor.
- People are abusing `NoopClientCall`, using it as an alternative of `mock(ClientCall.class)`, which is as bad as a mock.
- `TestMethodDescriptors.noopMarshaller` does not gain much benefit. It violates the interface's contract, it's equivalent to a `mock(Marshaller.class)`, and it does not work in InProcess transport or interop test.