Add support for expected objects to roundtrip test

[overhacked]

Add optional support to test the deserialized bytes against an expected static object. Uses a macro to concisely list modules that contain an expected() function that returns the expected Request or Response object.

Existing tests could have expected objects added to improve the error reporting in case of test failure. Tests from the proto::message module have been moved to tests/roundtrip because they were simply round-trip tests with an additional check against an expected object.

[overhacked]

Something weird here. I've run fmt twice and CI keeps failing on fmt --check. Will investigate today.

[wiktor-k]

You need a nightly formatter: cargo +nightly fmt for import reordering.

[overhacked]

🙃 I have been running cargo +nightly fmt, but I ran into a rustfmt bug, I think. I 1) ran fmt, 2) commit --amend the changes, then 3) push. CI fmt --check failed, and I discovered that if I ran cargo +nightly fmt again on the code I had committed, it had additional lints applied.

In this case, I had multiple trailing blank lines in a doc comment, and it was only stripping them off one line at a time. The next force-push I make should pass fmt --check.

[overhacked]

I ran into a rustfmt bug, I think.

Turned out to be true and submitted a fix: rust-lang/rustfmt#6163

[wiktor-k]

Wow, crazy! I didn't have time to look deeper in the code but it's cool that you've found a bug and submitted a fix PR 👏

[jcspencer]

These look great! Once this is merged I'm happy to go back and implement the same for the OpenSSH extensions we have built in, as they also implement their own roundtrip test logic; much nicer to make it more unified!

[overhacked]

Just broke this PR up into two separate commits to, hopefully, make it easier to review:

• The first commit contains the changes to tests/roundtrip infrastructure to support checking the deserialized object against an expected, statically-constructed object.
• The second commit is the conversion of proto::message unit tests to roundtrip tests, utilizing the new expected object support.
• (The third commit is a documentation fix to the first commit.)

[wiktor-k]

Okay, I've read it. I didn't expect it but it's an clever solution and the line-count is good so let's go ahead and merge it 🎉

Could you rebase on top of main? (If there are any conflicts it's because of my PRs and I'm happy to help resolve 😅 )

[overhacked]

Rebased onto main; should be good to go!

[wiktor-k]

Great! Thank you! 🙇‍♂️