tstest/integration/testcontrol: fix data race

Fix race from 1ec99e99f4

Fixes #3289

Change-Id: I58158d3f82339ac171fb14827c5f158d602327f4
Signed-off-by: Brad Fitzpatrick <bradfitz@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