Devise a way to test spans

[sanxiyn]

There are minimal tests in libsyntax unit tests, but they are 1. nowhere near comprehensive, 2. extremely painful to read and write.

[sanxiyn]

UI test (added in #33588) can test spans now.

Still, I think UI test can't test spans not used by diagnostics. Such spans can still affect rustfmt, for example.

[steveklabnik]

Triage: not aware of any changes

[estebank]

Current status:

• We have a way to test spans visual output
• We have a way to test the UI output for the entire test suite
• We have a way to test the code generated from applying suggestions (.fixed test files when adding // run-rustfix to test file)

@steveklabnik @sanxiyn do you think we can close this ticket given this?

[steveklabnik]

Sounds good to me. Let's close! <3