PR #82639 changed UI tests so `-Z unstable-options` aren't passed to UI
tests by default. This completely broke `use_suggestion_json.rs`, which
uses the unstable `--error-format=pretty-json` option. The expected
output went from 400 lines of pretty JSON error messages to a single
JSON error saying "`--error-format=pretty-json` is unstable"!
This commit adds `-Z unstable-options` back and reinstates the old
expected output, with some minor changes to account for shifted spans
and slightly JSON output changes since then.
