Rollup merge of #94126 - ssomers:alloc_prep_1, r=Mark-Simulacrum

Classify BinaryHeap & LinkedList unit tests as such

All but one of these so-called integration test case are unit tests, just like btree's were (#75531). In addition, reunite the unit tests of linked_list that were split off during #23104 because they needed to remain unit tests (they were later moved to the separate file they are in during #63207). The two sets could remain separate files, but I opted to merge them back together, more or less in the order they used to be, apart from one duplicate name `test_split_off` and one duplicate tiny function `list_from`.

Author: JohnTitor

library/alloc/src/collections/binary_heap.rs

@@ -155,6 +155,9 @@ use crate::vec::{self, AsVecIntoIter, Vec};
 
 use super::SpecExtend;
 
+#[cfg(test)]
+mod tests;
+
 /// A priority queue implemented with a binary heap.
 ///
 /// This will be a max-heap.

library/alloc/tests/binary_heap.rs renamed to library/alloc/src/collections/binary_heap/tests.rs

@@ -1,5 +1,5 @@
-use std::collections::binary_heap::{Drain, PeekMut};
-use std::collections::BinaryHeap;
+use super::*;
+use crate::boxed::Box;
 use std::iter::TrustedLen;
 use std::panic::{catch_unwind, AssertUnwindSafe};
 use std::sync::atomic::{AtomicU32, Ordering};