[test] Add ManualGCScope to test-spaces/SizeOfInitialHeap

The test does not expect GC to happen while it is running

Bug: v8:10988
Change-Id: Idcd30bde4ae1a7c3386a5d8c4c46e46e839e0fe9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2449971
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70316}

test/cctest/heap/test-spaces.cc

@@ -344,6 +344,7 @@ TEST(OldLargeObjectSpace) {
 // messages are also not stable if files are moved and modified during the build
 // process (jumbo builds).
 TEST(SizeOfInitialHeap) {
+  ManualGCScope manual_gc_scope;
   if (i::FLAG_always_opt) return;
   // Bootstrapping without a snapshot causes more allocations.
   CcTest::InitializeVM();