test: work around valgrind bug

test/test-async.c

@@ -50,6 +50,21 @@ void thread_cb(void *arg) {
 
     r = uv_async_send(&async);
     ASSERT(r == 0);
+
+    /* Work around a bug in Valgrind.
+     *
+     * Valgrind runs threads not in parallel but sequentially, i.e. one after
+     * the other. It also doesn't preempt them, instead it depends on threads
+     * yielding voluntarily by making a syscall.
+     *
+     * That never happens here: the pipe that is associated with the async
+     * handle is written to once but that's too early for Valgrind's scheduler
+     * to kick in. Afterwards, the thread busy-loops, starving the main thread.
+     * Therefore, we yield.
+     *
+     * This behavior has been observed with Valgrind 3.7.0 and 3.9.0.
+     */
+    uv_sleep(0);
   }
 }