Auto merge of #76391 - danii:master, r=cuviper

Split up core/test/iter.rs into multiple files

This PR removes the `// ignore-tidy-filelength` at the top of [iter.rs](https://github.com/rust-lang/rust/blob/04f44fb9232dc960d213d0df4a203c387215a5ff/library/core/tests/iter.rs) by splitting it into several sub files. I have split the file per test, based on what I felt the test was really trying to test.
Addresses issue #60302.
- [associated_util.rs](https://github.com/rust-lang/rust/blob/d29180a8ed2752c5dfa2bca1d28ca05dc9bf6bf1/library/core/tests/iter/associated_util.rs) - For testing the module level functions. (Maybe should be renamed?)
- [collection.rs](https://github.com/rust-lang/rust/blob/d29180a8ed2752c5dfa2bca1d28ca05dc9bf6bf1/library/core/tests/iter/collection.rs) - For testing methods that use the values of all the items in an iterator, and creates one value from them, whether it be a Vec of all the values, or a number that represents the sum.
- [mod.rs](https://github.com/rust-lang/rust/blob/d29180a8ed2752c5dfa2bca1d28ca05dc9bf6bf1/library/core/tests/iter/mod.rs) - For utility structs used in all tests, and other tests I didn't know where to place.
- [range.rs](https://github.com/rust-lang/rust/blob/d29180a8ed2752c5dfa2bca1d28ca05dc9bf6bf1/library/core/tests/iter/range.rs) - For testing ranges.
- [transformation.rs](https://github.com/rust-lang/rust/blob/d29180a8ed2752c5dfa2bca1d28ca05dc9bf6bf1/library/core/tests/iter/transformation.rs) - For testing methods that transform all the values in an iterator.
- [util.rs](https://github.com/rust-lang/rust/blob/d29180a8ed2752c5dfa2bca1d28ca05dc9bf6bf1/library/core/tests/iter/util.rs) - For testing methods that provide utility in more specific use cases.

"Programatically"
-----------------------
You may have noticed I "Programatically" split up the file. [This is how](https://gist.github.com/danii/a58b3bcafa9faf1c3e4b01ad7495baf4) I managed to do that. 😛 Hope that's fine.