net/connstats: mark TestConcurrent as flaky

Updates #7030

Change-Id: Ic46da5e5690b90b95028a68a3cf967ad86881e28
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