Adjust UI tests for `unit_bindings` lint
- Explicitly annotate `let x: () = expr;` where `x` has unit type, or remove the unit binding to leave only `expr;` instead.
- Use `let () = init;` or `let pat = ();` where appropriate.
- Fix disjoint-capture-in-same-closure test which wasn't actually testing a closure: `tests/ui/closures/2229_closure_analysis/run_pass/disjoint-capture-in-same-closure.rs`.
Note that unfortunately there's *a lot* of UI tests, there are a couple of places where I may have left something like `let (): ()` (this is not needed but is left over from an ealier version of the lint) which is bad style.
This PR is to help with the `unit_bindings` lint at #112380.
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