Change test suite atomic from SeqCst to Relaxed

This atomic is not synchronizing anything outside of its own value, so
we don't need the Acquire/Release guarantee that all memory operations
prior to the store are visible after the subsequent load, nor the SeqCst
guarantee of all threads seeing all of the sequentially consistent
operations in the same order.

Author: dtolnay

Diff for: tests/drop/mod.rs

@@ -2,8 +2,7 @@
 
 use std::error::Error as StdError;
 use std::fmt::{self, Display};
-use std::sync::atomic::AtomicBool;
-use std::sync::atomic::Ordering::SeqCst;
+use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::Arc;
 
 #[derive(Debug)]
@@ -19,7 +18,7 @@ impl Flag {
     }
 
     pub fn get(&self) -> bool {
-        self.atomic.load(SeqCst)
+        self.atomic.load(Ordering::Relaxed)
     }
 }
 
@@ -48,7 +47,7 @@ impl Display for DetectDrop {
 
 impl Drop for DetectDrop {
     fn drop(&mut self) {
-        let already_dropped = self.has_dropped.atomic.swap(true, SeqCst);
+        let already_dropped = self.has_dropped.atomic.swap(true, Ordering::Relaxed);
         assert!(!already_dropped);
     }
 }