Audit & Harden `run-pass` tests for `#![feature(bindings_after_at)]`

[Centril]

The feature bindings_after_at which allows for x @ Some(y) patterns, is implemented in #66296. The PR doesn't have as good coverage of the dynamic semantics as we might like. Once the PR lands, we should try to improve things before we stabilize the feature.

Notable tests to include:

• Dynamic drop (test(bindings_after_at): add dynamic drop tests for bindings_after_at #69810)

cc @pnkfelix @matthewjasper #65490

This issue has been assigned to @thekuom via this comment.

[Centril]

This is now unblocked as #66296 has landed.

[thekuom]

I would like to work on this one 👍

[Centril]

Go for it :)

@rustbot assign @thekuom