read method of ReaderBase child classes cannot be unit tested

[Saransh-cpp]

Because the methods accept an argument of type CLI::App*, and there is no public API to construct this object without running CLI + CLI11 provides no testing utilities. We test them in integration tests as of now, but it would be nice to somehow have unit tests for these / refactor them to use a custom object and not CLI::App. If time permits, otherwise future work.