Fixing multithreaded test

At some point I broke this test without realizing it. If you were lucky,
the test passed immediatly. If you were unlucky, integer underflow meant
the test would never finish.

Thankfully after fixing the logic issue with tracking outstanding work,
the test still passes -- thanks in part to this being tested in other
environments pretty thoroughly at this point.

tests/multithreaded.rs

@@ -37,6 +37,7 @@ fn round_robin() {
             .0
             .send(Command::Enqueue(Root::new(WorkUnit::default(), &guard)))
             .expect("worker disconnected early");
+        outstanding.push_one();
     }
     drop(guard);
 
@@ -76,7 +77,7 @@ fn thread_worker(
                 if queue.items.is_empty() {
                     next_thread
                         .send(Command::Work)
-                        .expect("next thread disconnted");
+                        .expect("next thread disconnected");
                 } else {
                     let work = match queue.items[0].as_root(&guard) {
                         Some(work) => {
@@ -119,6 +120,10 @@ impl OutstandingWork {
     fn complete_one(&self) {
         self.0.fetch_sub(1, Ordering::Acquire);
     }
+
+    fn push_one(&self) {
+        self.0.fetch_add(1, Ordering::Acquire);
+    }
 }
 
 struct WorkUnit {