Increase sleep time in sigwait64_SIGRTMIN test

With previous sleep time, if unlucky, newly started thread can send
signal before main thread starts sigwait.

Test: bionic-unit-tests --gtest_filter=signal.sigwait64_SIGRTMIN
Change-Id: I63d7825a695988e388903c7951e2435cb69773a1
diff --git a/tests/signal_test.cpp b/tests/signal_test.cpp
index 6db7751..3bc3135 100644
--- a/tests/signal_test.cpp
+++ b/tests/signal_test.cpp
@@ -176,7 +176,7 @@
 
   pid_t pid = getpid();
   std::thread thread([&pid]() {
-    usleep(5000);
+    sleep(1);
     kill(pid, SIGRTMIN);
   });