Start using a common helper for tests to declare that they require root.
This is step 1. A later step will then make this helper track which tests were
skipped so a subsequent pass will run these test as root.
Updates tailscale/corp#40007
Change-Id: I4979e1def0fa3691d38c83f48c89aaa443e7f62e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>