[Test] Avoid relying on flag implications for --flush-bytecode in tests.

Causes flakyness in TSAN runs when flag is written by EnforceFlagImplications
and read by ConcurrentMarking.

BUG=v8:8924

Change-Id: I2b0bf0fbb678e03492d7ed13e48657de9316b700
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1505796
Auto-Submit: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Mythri Alle <mythria@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60062}

test/mjsunit/compiler/dont-flush-code-marked-for-opt.js

@@ -2,7 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --opt --allow-natives-syntax --expose-gc --stress-flush-bytecode
+// Flags: --opt --allow-natives-syntax --expose-gc --flush-bytecode
+// Flags: --stress-flush-bytecode
 
 function foo(a) {}
 

test/mjsunit/regress/regress-v8-8799.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: --expose-gc --stress-flush-bytecode
+// Flags: --expose-gc --flush-bytecode --stress-flush-bytecode
 
 // Ensure tagged template objects are cached even after bytecode flushing.
 var f = (x) => eval`a${x}b`;