tstest: don't log on success

This is particularly important for benchmarks,
where stray output disrupts tooling.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f