Fix emit inconsitencies and add tests

[Sh3Rm4n]

While trying to convert the parsed payload of information elements into the binary format, I've noticed inconsistencies regarding some emit implementations.

I've fixed some of them (which I needed) mostly based on the assumption, that the Parsable implementation is correct.
On exception is the Nl80211RateAndSelector implementation, where the comments from this type and Nl80211Element::SupportedRatesAndSelectors where already inconsistent.

Also I've added some roudtrip test, using macros to reduce the repetition. I thought about adding more tests, but I wanted to propose this as a starting point for now.

[cathay4t]

Thanks for the patch! I think it is time to add unit test by parsing and emitting captured iw netlink packets, otherwise it is just your talk vs my talk.

[cathay4t]

https://github.com/rust-netlink/netlink-packet-route/?tab=readme-ov-file#development has steps to capture netlink packet. And that project also has unit test example on parsing and emitting packet.

[Sh3Rm4n]

If I'd split out the testing, we could at least merge the emit fixes. WDYT @cathay4t?

[cathay4t]

In general, it looks good. Just two cents:

1. helper.rs is to generic which will be bloated in the future like utils.rs. Maybe change it to emit_helper.rs.
2. For the buffer parsing part, could you take a look on https://github.com/nispor/mozim/blob/main/src/buffer.rs . I intend to replace buffer!() macros used in rust-netlink crates. Could you check whether that design also works in this nl80211 buffer parsing?

None of them is blocking the merge. Feel free to merge this PR if I am not responding.