tstest/integration/nat: add start of TestGrid

Updates #13038

Change-Id: I41d1c2bf20ae6dfbb071020d9dc2b742e7995835
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f