[test] Avoid disabling of RCS during runtime

Test was flaky since disabling of RCS at runtime is unsafe. Some code
(e.g. TRACE_GC) is run only in case RCS is enabled and such code paths
might also DCHECK that RCS is enabled.
A background thread (sweeping in this case) could've already
entered such a code path right before RCS is disabled. In this case the
guard at the entry still saw that RCS is enabled but subsequent DCHECKs
that ensure that RCS is enabled might fail.

We could fix this by completing the sweeper tasks before disabling RCS
but there might be other similar tasks which might be run at that point
in the future (e.g. memory unmapper).

Disabling the flag doesn't even seem to be needed for the initial
regression test, so it seems simpler to just not disable RCS anymore.

Bug: v8:12026
Change-Id: I878f7dd9a7a4abb6a501f7a7651a1240ef2082a6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3162043Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76845}

test/cctest/heap/test-heap.cc

@@ -7318,7 +7318,6 @@ TEST(Regress11181) {
   v8::HandleScope scope(CcTest::isolate());
   GenerateGarbage();
   CcTest::CollectAllAvailableGarbage();
-  TracingFlags::runtime_stats.store(0, std::memory_order_relaxed);
 }
 
 TEST(LongTaskStatsFullAtomic) {