[fuzzer] Flush std::cout after --wasm-fuzzer-gen-test

Change-Id: I2a49b2d0978c167c1ee69fbd64a36b9042aae571
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/5729412


Commit-Queue: Matthias Liedtke <mliedtke@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Auto-Submit: Matthias Liedtke <mliedtke@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#95149}