Switch from `trybuild` to `ui-test`

[joshlf]

See also: #560

The trybuild crate keeps track of expected compiler errors in a separate .stderr file - one .stderr file per .rs file. This makes it difficult to audit updates to these files, which tend to be very cluttered and verbose, and makes issues like #706 / #707 more likely to slip through.

By contrast, the ui-test crate allows annotating expected errors in an inline comment, which is much more readable and thus much less error-prone. It will also make it feasible for us to combine multiple tests in a single file without worrying about it increasing the likelihood of errors slipping through.

[joshlf]

This would likely have prevented #706 / #707.

[joshlf]

Another reason to migrate off of trybuild: dtolnay/trybuild#241 is preventing us from implementing a test which would detect a regression of #716.