chore: Apply Clippy lint collection_is_never_used

[jbencin]

Description

Apply Clippy lint collection_is_never_used. This found a lot of collections, mostly in test code, that were created but never read. These fell into a few different categories:

• Something which looked like it was intended to be used, but was not. These are possible bugs and should be reviewed by someone familiar with the code!
• Storing the result of calling a function with a side effect. I left these alone and marked them with #[allow(clippy::collection_is_never_read)]
• Obvious leftovers from copy/pasted blocks of code, particularly in tests

Note that collection_is_never_used is a "Nursery" lint, meaning it's not considered stable/accurate. It's not in the default set of lints Clippy runs, but still useful for finding unused code