std: move "mod tests/benches" to separate files

Also doing fmt inplace as requested.

library/std/src/sys_common/remutex/tests.rs

@@ -0,0 +1,72 @@
+use crate::cell::RefCell;
+use crate::sync::Arc;
+use crate::sys_common::remutex::{ReentrantMutex, ReentrantMutexGuard};
+use crate::thread;
+
+#[test]
+fn smoke() {
+    let m = unsafe {
+        let m = ReentrantMutex::new(());
+        m.init();
+        m
+    };
+    {
+        let a = m.lock();
+        {
+            let b = m.lock();
+            {
+                let c = m.lock();
+                assert_eq!(*c, ());
+            }
+            assert_eq!(*b, ());
+        }
+        assert_eq!(*a, ());
+    }
+}
+
+#[test]
+fn is_mutex() {
+    let m = unsafe {
+        let m = Arc::new(ReentrantMutex::new(RefCell::new(0)));
+        m.init();
+        m
+    };
+    let m2 = m.clone();
+    let lock = m.lock();
+    let child = thread::spawn(move || {
+        let lock = m2.lock();
+        assert_eq!(*lock.borrow(), 4950);
+    });
+    for i in 0..100 {
+        let lock = m.lock();
+        *lock.borrow_mut() += i;
+    }
+    drop(lock);
+    child.join().unwrap();
+}
+
+#[test]
+fn trylock_works() {
+    let m = unsafe {
+        let m = Arc::new(ReentrantMutex::new(()));
+        m.init();
+        m
+    };
+    let m2 = m.clone();
+    let _lock = m.try_lock();
+    let _lock2 = m.try_lock();
+    thread::spawn(move || {
+        let lock = m2.try_lock();
+        assert!(lock.is_none());
+    })
+    .join()
+    .unwrap();
+    let _lock3 = m.try_lock();
+}
+
+pub struct Answer<'a>(pub ReentrantMutexGuard<'a, RefCell<u32>>);
+impl Drop for Answer<'_> {
+    fn drop(&mut self) {
+        *self.0.borrow_mut() = 42;
+    }
+}