librarian: add end-to-end tests to verify copy functions in generate command

Add end to end tests to verify copy functions in generation command.

https://github.com/googleapis/librarian/blob/5456872712dc4ad2482cd1f25b1889349a71dec9/internal/librarian/generate.go#L272