test case to check opt_present() panic for undefined options #51.

[prataprc]

I am looking at issues to get a hang of rust lang and its ecosystem. If this commit does not make sense, please feel free to abandon. Thanks,

[KodrAus]

Thanks for the pull request @prataprc! This looks like a great start, I think we could refactor the test just a bit so it only hits the panic we expect when calling matches.opt_present:

match opts.parse(args) {
    Ok(matches) => assert!(!matches.opt_present("undefined")),
    Err(e) => println!("failed to parse opts: {}", e),
}

So that way if we fail to parse the args (even though we won't, and if we do then other tests should fail) we won't pass this one because there won't be any panic.

[prataprc]

Suggestion looks neat ! Amended the PR.

[KodrAus]

This looks good to me! Thanks @prataprc