Extend bindings_after_at (`b1 @ Some(b2)`) tests with nested `or_patterns`

[Centril]

Once #![feature(or_patterns)] (#54883), i.e. pat_0 | ... | pat_n in nested positions, e.g. Some(Ok(a) | Err(b)) have progressed sufficiently in their implementation, e.g. when the MIR building implementation is done, we should extend the test suite for #![feature(bindings_after_at)] (#65490), e.g. b1 @ Some(b2), to exercise the combination of these two features. Ideally, we would also account for the combination of those two with #![feature(slice_patterns)] (#62254) and #![feature(box_patterns)] (#29641) as well to make things really air-tight.

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

[Centril]

This is almost unblocked now that #66296 has landed. However, we need more progress on or-patterns.

[thekuom]

Can I work on this one?

[Centril]

Sure, go right ahead.

@rustbot assign @thekuom

[rustbot]

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