[wasm][fuzzer] Run micro tasks after each fuzzer execution

This asserts that we run e.g. GC finalization tasks. Without that, we
might run ouf of memory.

R=ahaas@chromium.org

Bug: chromium:938739
Change-Id: Ic80074f877183bcabb3353fbeff94842a534efab
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1526007Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60266}

test/fuzzer/wasm.cc

@@ -55,5 +55,9 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
   if (compiles) {
     i::wasm::fuzzer::InterpretAndExecuteModule(i_isolate, module_object);
   }
+
+  // Pump the message loop and run micro tasks, e.g. GC finalization tasks.
+  support->PumpMessageLoop(v8::platform::MessageLoopBehavior::kDoNotWait);
+  isolate->RunMicrotasks();
   return 0;
 }