6.0: allow expect in webauthn-authenticator-rs, and in all tests

[micolous]

The master branch currently allows expect() to be used anywhere, but the 6.0 branch doesn't.

In the longer term, we should fix these.

But denying expect() currently triggers CI build failures with certain features enabled (cable, mozilla, softtoken, ui-cli, win10), which aren't enabled by default.

Some of these uses will be addressed as we rewrite to drop OpenSSL support for #499, but with clippy being an early step in CI, additional failures make it impossible to measure progress using the CI build matrix (ie: feature combinations becoming or returning to green).

I've also allowed expect() in tests, like unwrap() is allowed. Using expect() or unwrap() gives us stack traces for unexpected errors in tests, so it's much easier to trace how a test broke.

This takes us from 11/35 to 19/35 passing tests.

• cargo test has been run and passes
• documentation has been updated with relevant examples (if relevant)

[yaleman]

How many uses of expect are there in the non-test code? normally we just allow-list them on a case-by-case basis to indicate it's been checked etc.

Test code it's fine to use, since it gives feedback unlike unwrap 😄

[Firstyear]

Yep, this is fine shortterm.