Fix test usage of pollset

test/core/iomgr/workqueue_test.c

@@ -73,8 +73,10 @@ static void test_add_closure(void) {
 
   gpr_mu_lock(g_mu);
   GPR_ASSERT(!done);
-  grpc_pollset_work(&exec_ctx, g_pollset, &worker, gpr_now(deadline.clock_type),
-                    deadline);
+  while (!done) {
+    grpc_pollset_work(&exec_ctx, g_pollset, &worker,
+                      gpr_now(deadline.clock_type), deadline);
+  }
   gpr_mu_unlock(g_mu);
   grpc_exec_ctx_finish(&exec_ctx);
   GPR_ASSERT(done);
@@ -97,9 +99,10 @@ static void test_flush(void) {
   grpc_workqueue_add_to_pollset(&exec_ctx, wq, g_pollset);
 
   gpr_mu_lock(g_mu);
-  GPR_ASSERT(!done);
-  grpc_pollset_work(&exec_ctx, g_pollset, &worker, gpr_now(deadline.clock_type),
-                    deadline);
+  while (!done) {
+    grpc_pollset_work(&exec_ctx, g_pollset, &worker,
+                      gpr_now(deadline.clock_type), deadline);
+  }
   gpr_mu_unlock(g_mu);
   grpc_exec_ctx_finish(&exec_ctx);
   GPR_ASSERT(done);