Upgrade UI tests to 2021

[Manishearth]

We should upgrade our UI tests to Rust 2021

This mostly involves adding a // edition:2021 up top and ensuring the test still passes.

It's worth splitting this task up.

[Manishearth]

An example of something broken due to this: #7723

[camsteffen]

Could we change this at the test suite level instead?

[Manishearth]

Yeah @llogiq is working on that

(Sorry, I haven't been able to update issues well, I've been mostly on my phone)

[Manishearth]

Manishearth/compiletest-rs#246

[llogiq]

I can already report back that only a handful of tests fail with the new edition. Working through them now as I find the time.