Fix paths to coretests and alloctests

Following `#135937` and `#136642`, tests for core and alloc are in
coretests and alloctests. Update paths referring to the old locations.

Some context for changes which don't match that pattern:
* library/std/src/thread/local/dynamic_tests.rs and
  library/std/src/sync/mpsc/sync_tests.rs were moved under
  library/std/tests/ in 332fb7e (Move std::thread_local unit tests
  to integration tests, 2025-01-17) and b8ae372 (Move std::sync unit
  tests to integration tests, 2025-01-17), respectively, so are no
  longer special cases.
* There never was a library/core/tests/fmt.rs file. That comment
  previously referred to src/test/ui/ifmt.rs, which was folded into
  library/alloc/tests/fmt.rs in 949c966 (move format! interface
  tests, 2020-09-08).

Author: thaliaarchi

compiler/rustc_codegen_gcc/build_system/src/test.rs

@@ -677,7 +677,7 @@ fn test_projects(env: &Env, args: &TestArg) -> Result<(), String> {
 fn test_libcore(env: &Env, args: &TestArg) -> Result<(), String> {
     // FIXME: create a function "display_if_not_quiet" or something along the line.
     println!("[TEST] libcore");
-    let path = get_sysroot_dir().join("sysroot_src/library/core/tests");
+    let path = get_sysroot_dir().join("sysroot_src/library/coretests/tests");
     let _ = remove_dir_all(path.join("target"));
     run_cargo_command(&[&"test"], Some(&path), env, args)?;
     Ok(())

library/core/src/fmt/mod.rs

@@ -3016,6 +3016,5 @@ impl<T: ?Sized> Debug for SyncUnsafeCell<T> {
     }
 }
 
-// If you expected tests to be here, look instead at the core/tests/fmt.rs file,
+// If you expected tests to be here, look instead at the alloctests/tests/fmt.rs file,
 // it's a lot easier than creating all of the rt::Piece structures here.
-// There are also tests in the alloc crate, for those that need allocations.

library/coretests/tests/str.rs

@@ -1 +1 @@
-// All `str` tests live in library/alloc/tests/str.rs
+// All `str` tests live in library/alloctests/tests/str.rs

src/tools/tidy/src/unit_tests.rs

@@ -1,11 +1,13 @@
-//! Tidy check to ensure `#[test]` and `#[bench]` are not used directly inside `core`.
+//! Tidy check to ensure `#[test]` and `#[bench]` are not used directly inside
+//! `core` or `alloc`.
 //!
 //! `#![no_core]` libraries cannot be tested directly due to duplicating lang
-//! items. All tests and benchmarks must be written externally in `core/{tests,benches}`.
+//! items. All tests and benchmarks must be written externally in
+//! `{coretests,alloctests}/{tests,benches}`.
 //!
-//! Outside of core tests and benchmarks should be outlined into separate files
-//! named `tests.rs` or `benches.rs`, or directories named `tests` or `benches` unconfigured
-//! during normal build.
+//! Outside of `core` and `alloc`, tests and benchmarks should be outlined into
+//! separate files named `tests.rs` or `benches.rs`, or directories named
+//! `tests` or `benches` unconfigured during normal build.
 
 use std::path::Path;
 
@@ -14,40 +16,43 @@ use crate::walk::{filter_dirs, walk};
 pub fn check(root_path: &Path, bad: &mut bool) {
     let core = root_path.join("core");
     let core_copy = core.clone();
-    let core_tests = core.join("tests");
-    let core_benches = core.join("benches");
-    let is_core = move |path: &Path| {
-        path.starts_with(&core)
-            && !(path.starts_with(&core_tests) || path.starts_with(&core_benches))
-    };
+    let is_core = move |path: &Path| path.starts_with(&core);
+    let alloc = root_path.join("alloc");
+    let alloc_copy = alloc.clone();
+    let is_alloc = move |path: &Path| path.starts_with(&alloc);
 
     let skip = move |path: &Path, is_dir| {
         let file_name = path.file_name().unwrap_or_default();
         if is_dir {
             filter_dirs(path)
                 || path.ends_with("src/doc")
-                || (file_name == "tests" || file_name == "benches") && !is_core(path)
+                || (file_name == "tests" || file_name == "benches")
+                    && !is_core(path)
+                    && !is_alloc(path)
         } else {
             let extension = path.extension().unwrap_or_default();
             extension != "rs"
-                || (file_name == "tests.rs" || file_name == "benches.rs") && !is_core(path)
-                // UI tests with different names
-                || path.ends_with("src/thread/local/dynamic_tests.rs")
-                || path.ends_with("src/sync/mpsc/sync_tests.rs")
+                || (file_name == "tests.rs" || file_name == "benches.rs")
+                    && !is_core(path)
+                    && !is_alloc(path)
         }
     };
 
     walk(root_path, skip, &mut |entry, contents| {
         let path = entry.path();
         let is_core = path.starts_with(&core_copy);
+        let is_alloc = path.starts_with(&alloc_copy);
         for (i, line) in contents.lines().enumerate() {
             let line = line.trim();
             let is_test = || line.contains("#[test]") && !line.contains("`#[test]");
             let is_bench = || line.contains("#[bench]") && !line.contains("`#[bench]");
             if !line.starts_with("//") && (is_test() || is_bench()) {
                 let explanation = if is_core {
                     "core unit tests and benchmarks must be placed into \
-                         `core/tests` or `core/benches`"
+                         `coretests/tests` or `coretests/benches`"
+                } else if is_alloc {
+                    "alloc unit tests and benchmarks must be placed into \
+                         `alloctests/tests` or `alloctests/benches`"
                 } else {
                     "unit tests and benchmarks must be placed into \
                          separate files or directories named \