tstest/natlab: move running instructions to a README

Updates #cleanup

Change-Id: I9580fbfeeef2edf0c678c8ab1ec70131fd20f0b5
Signed-off-by: Francois Marier <francois@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