Disable --stress-opt in Liftoff tier down test

With --stress-opt, the second run will share the NativeModule with the
first run, hence it's in a nondeterministic state and the test
expectations fail.

TBR=ahaas@chromium.org
CC=​​duongn@microsoft.com

No-Try: true
Bug: v8:10086, v8:9654
Change-Id: I74cf5e841ae2330b3b846ee742cc022305ec9636
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2000750
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65773}

test/mjsunit/wasm/tier-down-to-liftoff.js

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --allow-natives-syntax --liftoff --wasm-tier-up
+// Flags: --allow-natives-syntax --liftoff --wasm-tier-up --no-stress-opt
 
 load('test/mjsunit/wasm/wasm-module-builder.js');