Additional test for uN::{gather,scatter}_bits

[quaternic]

Feature gate: #![feature(uint_gather_scatter_bits)]
Tracking issue: #149069
Accepted ACP: rust-lang/libs-team#695 (comment)

Adds an additional runtime test for uN::gather_bits and uN::scatter_bits in coretests. They are each other's inverses in a sense, so a shared test can test both with relative ease.

I plan to follow up with optimized implementations for these functions.

[rustbot]

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[Mark-Simulacrum]

@bors r+ rollup

[bors]

📌 Commit f49eaec has been approved by Mark-Simulacrum

It is now in the queue for this repository.