net/speedtest: mark flaky test, and skip it by default as it's slow

Updates #17338

Change-Id: I1f3dbc154ba274f615cc77d2aa76f6ff9d40137c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f