Rollup merge of rust-lang#136642 - bjorn3:separate_alloctest_crate, r=cuviper

Put the alloc unit tests in a separate alloctests package

Same rationale as rust-lang#135937. This PR has some extra complexity though as a decent amount of tests are testing internal implementation details rather than the public api. As such I opted to include the modules containing the types under test using `#[path]` into the alloctests package. This means that those modules still need `#[cfg(test)]`, but the rest of liballoc no longer need it.

Author: jhpratt

Cargo.lock

@@ -4,6 +4,7 @@ members = [
   "std",
   "sysroot",
   "coretests",
+  "alloctests",
 ]
 
 exclude = [