add tests differing between stable and new rules (with errors on new rules)

Since there are so many ways to write these, I've opted to only include
two sorts of test: simple tests that directly target the rules differing
between rulesets and nuanced tests that produce different errors under
different rulesets. I've also tried not to add any duplicate tests.

`well-typed-edition-2024.rs` already has tests disagreeing with stable,
so I've opted not to include any in this commit that are well-typed
under the new rules.

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