[test] Fix setting flags in SingleThreadedDefaultPlatformTest

We will not be able to modify flags after initializing V8 (soon).
The {SingleThreadedDefaultPlatformTest} was resetting flags during
teardown for no reason, as we do not support running multiple tests in a
row anyway. Thus remove that use of {SaveFlags} and just set the
--single-threaded flag before initializing V8.

R=ahaas@chromium.org

Bug: v8:12887
Change-Id: Ia89d442cf4b2fe2e12e258da5c0c9f1f871ded12
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3695378Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81024}

test/unittests/libplatform/single-threaded-default-platform-unittest.cc

@@ -40,26 +40,16 @@ class SingleThreadedDefaultPlatformTest
                   ::testing::Test>>> {
  public:
   static void SetUpTestSuite() {
-    CHECK_NULL(save_flags_);
-    save_flags_ = new i::SaveFlags();
-    v8::V8::SetFlagsFromString("--single-threaded");
+    i::FLAG_single_threaded = true;
+    i::FlagList::EnforceFlagImplications();
     WithIsolateScopeMixin::SetUpTestSuite();
   }
 
   static void TearDownTestSuite() {
     WithIsolateScopeMixin::TearDownTestSuite();
-    CHECK_NOT_NULL(save_flags_);
-    delete save_flags_;
-    save_flags_ = nullptr;
   }
-
- private:
-  static i::SaveFlags* save_flags_;
 };
 
-// static
-i::SaveFlags* SingleThreadedDefaultPlatformTest::save_flags_;
-
 TEST_F(SingleThreadedDefaultPlatformTest, SingleThreadedDefaultPlatform) {
   {
     i::HandleScope scope(i_isolate());