We can observe a data race in tests when logging after a test is
finished. `b.onHealthChange` is called in a goroutine after being
registered with `health.Tracker.RegisterWatcher`, which calls callbacks
in `setUnhealthyLocked` in a new goroutine.
See: https://github.com/tailscale/tailscale/actions/runs/9672919302/job/26686038740
Updates #12054
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ibf22cc994965d88a9e7236544878d5373f91229e